Electrode array and deployment assembly including an electrode array that is folded into a cannula that is narrower in width than the array

ABSTRACT

An electrode array assembly with a frame that is foldable or bendable on which electrodes are disposed. The frame includes laterally spaced apart bridges. Tabs extend laterally outwardly from the bridges Electrodes are disposed on the tabs. Beams, also part of the frame, extend between the laterally adjacent bridges. The frame can be folded around the beams so as to cause the laterally spaced bridges to at least partially overlap. When the beams are so bent, the electrode array can be disposed in an access cannula that has a diameter less than the width of the unfolded array.

RELATIONSHIPS TO EARLIER FIELD APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/873,397 filed 1 Sep. 2010 now U.S. Pat. No. 8,560,083. ApplicationSer. No. 12/873,397 is a continuation of PCT Pat. App. No.PCT/US2009/033769 filed 11 Feb. 2009. PCT Pat. App. No.PCT/US2009/0033769 is a nonprovisional application that claims priorityfrom U.S. Pat. App. No. 61/034,367 filed 6 Mar. 2008 and No. 61/139,395filed 19 Dec. 2008. The contents of the application from which thisapplication claims priority are now explicitly incorporated herein byreference.

FIELD OF THE INVENTION

This invention relates generally to an electrode array, with multipleelectrodes, that can be minimally invasively implanted adjacent targettissue within a patient and a tool for implanting the electrode array.

BACKGROUND OF THE INVENTION

There are a number of medical conditions for which it has been foundthat an effective therapy involves driving current through a section ofthe tissue of a patient. Often, the current is driven between theelectrodes of an electrode array implanted in the patient. Generally,the electrode array includes a non-conductive frame on which typicallytwo or more electrodes are disposed. Once the electrode array isimplanted, current is driven from at least one of the electrodes,through the adjacent tissue, to at least one of the other electrodes.The current flow through the tissue influences the tissue to accomplisha desired therapeutic result. For example, an electrode array positionedadjacent the heart may flow currents to stimulate the appropriatecontraction and expansion of the heart muscles. There is an increasinginterest in implanting electrode arrays adjacent neurological tissue sothat the resultant current flow induces a desired neurological orphysical effect. In one known application, the current driven betweenthe electrodes of such an array reduces the extent to which chronic painsignals are sent to the brain. This type of therapy is sometimesreferred to as neuromodulation for pain management. Alternatively, thecurrent flow stimulates a feeling of stomach fullness as part of anappetite suppression/weight management therapy. In another application,the current is flowed to tissue or nerves associated with the bladder orthe anal sphincter to assist in control of incontinence. Electrodes maybe implanted in a paralysis victim to provide muscle control and/or asense of feeling.

Often, the current driven between the electrodes is sourced from a pulsegenerator also implanted in the patient, known as an implantable pulsegenerator (IPG).

Once the electrode array of the above assembly is implanted, current canbe driven between selective sets of electrodes. By convention, oneelectrode is often referred to as the anode and the complementaryelectrode the cathode. In practice, since the current is typically abiphasic current; at one instant during the operation of the electrodearray a particular electrode may function as an anode and, in the nextinstant a cathode.

The term “cathode” is typically associated with the electrode that,during the first phase of a biphasic pulse serves as a current source.The term “anode” is typically associated with the electrode that, duringthe first phase of a biphasic pulse serves as a current sink.

There are a number of different protocols for driving current betweenthe electrodes of an array. One protocol is monopolar stimulation.During monopolar stimulation, current is driven from/to one or aplurality of electrodes on an electrode array to/from the case of theimplanted pulse generator. The IPG thus functions as the complementaryelectrode to/from which the current is driven. Given the large surfacearea of the case, and it's relatively large distance away from theimplanted electrode/electrodes, the current flowing through the tissueadjacent the case is very weak. Consequently, this current essentiallyhas no effect on this tissue.

Alternatively, the array may be operated in a bipolar mode. In thebipolar mode, two of the electrodes on the array serve as thecomplementary electrodes between which the current is driven. Whencurrent is driven between three electrodes, the array is considered tobe operating in a tripolar mode. Typically, when current is driven inthe tripolar mode, there is a center electrode and two outer electrodes.At any given instant, the charge of the two outer electrodes is of afirst polarity while the charge of the center electrode is of a second,opposite polarity.

When an array is operating in either the bipolar or tripolar mode, thecurrent flow between the electrodes is, in comparison to monopolarstimulation, more focused. This allows more selective targeting of thetissue through which the current is to be flowed.

In bipolar or tripolar systems, the case of the IPG often functions as aneutral or reference electrode. Cathodal and anodal pulses are,respectively of negative and positive potential relative to the IPGcase.

It is further understood that, regardless of whether the array isoperated in the monopolar, bipolar or tripolar mode the array isoperated so that charge balancing occurs. This means that the electrodesare operated so that, at any given instant, the current sourced by someof the electrodes is the same current that is sunk by the complementaryelectrodes. This charge balancing substantially eliminates the flow ofleakage current through tissue away from the target tissue to the IPGcase. Charge balancing therefore reduces the potentially adverse effectsof such leakage current.

Further, it is known to operate the electrode array assembly so that thecharge around each electrode is balanced on a pulse-by-pulse basis. Thischarge balancing around each electrode is performed to prevent a netdirect current from being applied to the tissue adjacent the array andthe electrodes themselves. This direct current has been known to damageboth electrodes as well as the tissue adjacent the electrodes.

This balance can be performed either passively or actively. Passivebalancing is achieved by placing a capacitor in series with theelectrode to store/draw residual polarized charge local to thetissue-electrode interface away from the tissue. The capacitoreffectively prevents the passage of a net direct current. However, toensure this balancing, the capacitor must be given sufficient time todischarge. During this discharge period, there may be sufficient currentflow that either the electrode or underlying tissue could be damaged.

Active charging balancing is achieved by applying biphasic stimulationpulses to an electrode. In this method, the charges sourced during theopposed phases of the pulse must be equal to each other to achieve totalcharge balance on the face of the electrode.

In the active charge balancing processes, the initial phase of thebiphasic stimulation process is referred to as the lead phase. Thesecond phase is referred to as the balancing phase. Often the magnitudeof the current flow during the balancing phase is less than that ofcurrent flow during the lead phase. Sometimes, the magnitude of thecurrent flow during the balancing phase is so low, it does not trigger aresponse in the tissue through which the current is flowed. The currentflowed during the balancing phase thus serves only to achieve thedesired individual electrode charge balancing.

Many electrode arrays have more than three electrodes. Once an array isimplanted, the current is initially driven between different sets ofelectrodes. The goal of this experimentation is to find the current paththrough the tissue that results in the most benefit and/or least adverseside effects to the patient. This can result in the array operating in astate in which one or more electrodes neither serve as current sourcesor current sinks.

Many electrode assemblies used in pain management therapy procedures andother therapies are shaped like flexible, elongated rods. This type ofelectrode assembly has a diameter typically no greater than 2 mm. Atleast for pain management applications, the electrode assembly is sosized so it can be positioned in the epidural space within a vertebralcolumn, the space between the ligamentum flavum and the spinal corddura. More particularly the electrode assembly is positioned in thisspace on or near the spinal cord dura. The electrode assembly issufficiently miniaturized so it can be introduced through a cannula orneedle-like delivery device. This eliminates the need to invasively cutthrough the paraspinal muscles, interspinus ligament, ligamentum flavumand expose portions of the lamina to gain access to the epidural spaceto position the electrode. The electrode assembly itself includes anumber of longitudinally spaced apart electrodes. Once the electrodeassembly is positioned adjacent the dura, current pulses are appliedbetween selected sets of electrodes. These current pulses flow, in part,through the spinal cord. The electrode current flow patterns areexperimented with until the individual reports, instead of pain, a moretolerable tingling sensation. This tingling sensation is known asparesthesia.

The above therapy offers some relief to many individuals suffering fromchronic pain. One disadvantage of these assemblies is that theirconstruction provides a relatively low spatial resolution of spinal cordstimulation. If the current pulses cannot be directed through thesections of the dorsal column that are most closely associated with theneurons through which the pain signals are being transmitted, theapplication of the signal may not result in appreciable masking of thepain signals. One of the few ways to compensate for this imprecisetargeting is to over stimulate the dorsal column nerves. This may resultin some nerves being subjected to needless stimulation. Moreover, thepotential fields generated between individual electrodes tend to drivecurrents radially. The emission of a fraction of this current away fromthe spinal cord is needless expenditure of electrical energy. This canbe a significant drawback in an implanted device in which the availablepower is limited. Also, owing to the rounded and flexible shape of thistype of electrode assembly, it can shift position along the spinal cord.Should this event occur, the current pulses directed between theindividual electrodes may no longer be of any use for inhibiting thetransmission of pain signals.

There have been attempts to overcome some of the above limitations byproviding implantable electrode systems that include electrode arraysthat are paddle-shaped. The electrodes integral with a paddle shapedelectrode array are typically spaced closer together than the electrodesof a rod type electrode array assembly. Further, the electrodes of apaddle-type electrode array are typically positioned towards the surfaceof the dura. To implant this type of assembly, it is necessary toinvasively cut through the para-spinal muscles, interspinus ligament,ligamentum flavum and portions of the lamina. This implantation requiresa relatively invasive surgical procedure.

SUMMARY OF THE INVENTION

This invention is directed to a new and useful electrode array assemblyadapted for implantation against or within living tissue. The electrodearray assembly of this invention includes a frame on which pluralspaced-apart electrodes are arranged in rows and columns. The frame canbe shaped to have a curved profile. The frame of the electrode arrayassembly of this invention is further designed to be foldable withoutsustaining substantial permanent deformation. Once unfolded, the framereturns to its unfolded, curved, profile.

Owing to the arrangement of the electrodes in the row by column array,the electrodes of this invention are disposed over an area that has bothlength and width. When the assembly is positioned over tissue, forexample, the spinal cord dura, current pulses can be directed throughthe underlying tissue in a number of different paths. This increases thelikelihood that current will flow through and therefore activatespecific nerves in the dorsal column that will appreciably modulateundesired pain signals.

The curved shape of the frame means that the electrode assembly as awhole has a curved shape. The curved shaped of the electrode assemblycauses the assembly to conform relatively closely to the tissue againstwhich it is placed. This conformance minimizes the likelihood that theassembly will move from the implanted position.

The electrode array assembly of this invention, in addition topotentially being curved, can be folded. To position the electrode arrayassembly of this invention into the body, against tissue, the assemblycan be first folded so it fits into the lumen of an introducer cannulaor introducer needle. The tip of the introducer cannula/needle ispositioned adjacent the location where the electrode assembly is to belocated. Once the cannula/needle is so positioned, the electrode arrayassembly is ejected. Upon ejection from the cannula/needle, theelectrode array assembly unfolds to take its defined shape against thetissue against which it is to be positioned.

It should be appreciated that the electrode array assembly of thisinvention has numerous electrodes that are disposed over a relativelylarge surface area. Once deployed, the current can be sourced from afirst set of electrodes and sunk to a second set of electrodes.Switching between which electrodes the current is flowed, invariablyshifts through which tissue the current is flowed. The practitioner can,by experimentation, adjust through which electrodes the current isflowed as well as the magnitude of current flow, to determine which anintra-tissue current flow results in the most desirable therapeuticbenefits and/or tolerable side effects. Once this current flow path isestablished, the electrode array assembly of this invention can be setso the electrodes continually source and sink current that results inthis optimal intra-tissue current flow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an electrode array assembly of thisinvention disposed against the dura surrounding the spinal cord;

FIG. 2 is a plan view of the electrode array assembly of this inventionlooking at the surface of the assembly on which the electrodes aredisposed;

FIG. 2A is an enlarged view of a section of the electrode array assemblyseen in FIG. 2;

FIG. 3 is a cross sectional view of the electrode array taken along aportion of the substrate at which two conductive traces are present;

FIG. 4 is a cross sectional view of one of the electrodes of theelectrode array assembly;

FIGS. 5-17 are a sequence of cross sectional views depicting how asubstrate containing the electrodes of this assembly are fabricated on awafer;

FIG. 18 is a flow chart of the initial process steps executed to formthe frame;

FIG. 19 is a plan view of frame

FIG. 20 is a cross sectional view of the curved and coated frame;

FIG. 21 depicts how the frame and substrate of this invention arepositioned in an aligning jig;

FIG. 22 depicts how the frame and substrate of this invention arepositioned in a press assembly;

FIG. 23 is time line representing the stages of the press sequence;

FIG. 24 depicts how the electrode array assembly is positioned betweenthe wires of a folding loom and how the loom wires are displaced to foldthe assembly;

FIG. 25 depicts the folded electrode array assembly of this invention;

FIG. 26 illustrates how a folded electrode array assembly of thisinvention is disposed in a cannula;

FIG. 27 depicts how the folded electrode array assembly disposed in theinsertion tool comprising an inner cannula and an outer cannula;

FIG. 28 is a cross sectional view of the tool and electrode arrayassembly of FIG. 27

FIG. 29 is a side view illustrating the initial placement of theinsertion tool, with the electrode array assembly disposed thereinadjacent a section of the spinal cord dura against which the assembly isto be positioned;

FIG. 30 is a cross sectional view of tool and electrode array assemblyillustrating the orientation of the assembly relative to the surface ofthe dura against which the assembly is to be positioned;

FIG. 31 is a side view illustrating the longitudinal advancement of theinner cannula and electrode array assembly out of the outer cannula;

FIG. 32 is a cross sectional view illustrating the rotation of the innercannula and electrode array assembly contained therein toward thedeployed orientation;

FIG. 33 is a cross sectional view illustrating the inner cannula andelectrode array assembly contained therein fully rotated to the deployedorientation relative to the surface of the dura against which theelectrode array assembly is to be deployed;

FIG. 34 is a side view illustrating the retraction of the inner cannulaback into the outer cannula so as to result in the deployment of theelectrode array assembly over the dura.

FIG. 35 is a plan view of an alternative electrode array assembly ofthis invention;

FIG. 36 is an enlarged plan view of the proximal end of the electrodearray of FIG. 35 wherein the drive module is absent from the terminalpad;

FIG. 37 is a cross sectional view across the width of a single electrodeof the assembly of this invention;

FIG. 38 is a cross sectional view across two conductors of the assemblyof this invention;

FIG. 39 is a side view of the proximal end of the electrode arrayassembly showing how the assembly has a curved profile;

FIG. 40 is a cross sectional view of the terminal pad of the assemblywith drive module removed to allow identification of the features of theterminal pad;

FIG. 41 is a cross sectional view of the terminal pad showing how thedrive module is fitted to the pad;

FIG. 42 is a block diagram of a number of the sub circuits that may beinternal to the drive module;

FIG. 43 is a perspective view of the electrode array off FIG. 35 in thefolded state;

FIG. 44 is view located forward from the proximal end of the electrodearray assembly when the assembly is in the folded state;

FIG. 45 is a top plan view of the electrode array when in the foldedstate;

FIG. 46 is a perspective view of the electrode array of FIG. 35 deployedover a section of spinal cord dura;

FIG. 47A is a key for FIGS. 47B through 47E indicating which of set ofeight electrodes are, at a given instant, employed as source or sinkelectrodes;

FIGS. 47B through 47E are diagrammatic representations of how differentsets of electrodes forming the array of this invention aresimultaneously actuated to serve as source or sink electrodes;

FIG. 48 represents the current density in the tissue when the electrodesare actuated according to the pattern of FIG. 47B;

FIG. 49 represents the current density of the tissue when the electrodesare actuated according to the pattern of FIG. 47C;

FIG. 50 represents the current density of the tissue when the electrodesare actuated according to the pattern of FIG. 47D;

FIG. 51 represents the current density of the tissue when the electrodesare actuated according to the pattern of FIG. 47E;

FIG. 52 is a diagrammatic illustration of how, by sourcing and sinkingcurrent between different electrodes of the two dimensional version ofthe assembly of this invention, current can be focused through tissuethat is off line from a particular column of electrodes; and

FIG. 53 is a diagrammatic illustration of how, by sourcing and sinkingcurrent between the electrodes of separate sets of electrodes, currentcan be simultaneously flowed through three, four or more sections oftissue with the assembly of this invention.

DETAILED DESCRIPTION I. Electrode Array Assembly

FIG. 1 is a perspective view of an electrode array assembly 28 of thisinvention disposed over a dura 31 that surrounds a spinal cord 30. Asbest seen in FIGS. 2-4, electrode array assembly 28 includes a frame 32,seen in FIGS. 3 and 4. Frame 32 is formed from a thin section ofsuperelastic material that can be both formed and folded into non-linearshapes. Frame 32 supports a non-conductive substrate 34. Substrate 34generally covers the whole of the surface of the frame 32. A number ofspaced apart electrodes 36 are disposed on the substrate 34.

In the illustrated version of the invention, each electrode 36 is theshape of a rectangle with rounded corners.

In FIG. 2A, for ease of illustration only four electrodes, theelectrodes in the first column of electrodes (the column on the leftside of the drawing) are illustrated. Also, the positions of theelectrodes in the fifth column of electrodes (the column on the rightside of the drawing), are illustrated as dashed rectangles. In someversions of the invention, it is contemplated that each electrode 36 hasa length of between 250 and 6000 microns and more typically between 1000and 3000 microns. Each electrode 36 has a width between 100 and 2000microns and typically between 500 to 1500 microns. In many versions ofthe invention, the minimal spacing between electrodes 36 should be atleast 100 microns. In some versions of the invention, the separationbetween the electrodes is at least 300 microns. Electrodes 36 arearranged in plural spaced apart rows and spaced apart columns on thesubstrate 34.

In the illustrated version of the invention, the electrodes 36 in eachcolumn of electrodes are aligned along a common longitudinal axis. Theelectrodes 36 in each row of electrodes are not so aligned. Thus, thelateral axes of adjacent electrodes, the electrodes in adjacent columns,are offset from each other. In the version of the invention depicted inFIG. 2, the lateral axes of the electrode rows in the first, third andfifth columns are aligned. The lateral axes of the electrode rows in thesecond and fourth columns are also aligned with each other.

A conductive trace 38 extends to each electrode 36. In FIG. 2A, for easeof illustration, only the conductive traces 38 and complementaryconductive fans 39 of two of the illustrated electrodes 36 are shown. Inthe illustrated version of the invention, the end of each conductivetrace 38 closest to the electrode 36 with which the trace 38 isassociated widens into a conductive fan 39. Each conductive fan 39 has alongitudinal edge contiguous with the electrode 36 with which the fan isassociated.

In the illustrated version of the invention, the electrode arrayassembly 28 has a head 40 with a relatively wide width. Head 40 isformed to have an outwardly curved front end 35. Electrodes 36 aredisposed on the assembly head 40. A number of spaced apart legs 41extend rearwardly from the assembly head 40. In the illustrated versionof the invention, the electrode array assembly has five (5) legs 41 a-e.The center leg 41 c is straight, at least parallel to if not alignedalong the longitudinal axis of the assembly head 40. The outer legs 41a, 41 b, 41 d and 41 e taper inwardly towards center leg 41 c. Legs 41terminate at a common foot 43 also part of the electrode array assembly28. Foot 43, which may be longitudinally axially aligned with head 40,has a width less than the width of the head.

Disposed on the surface of the assembly foot 43 is an antenna 44 and apair of circuits 45. In some versions of the invention, antenna 44 andcircuits 45 are disposed on the same side of the frame on whichelectrodes 36 are located. This is not required in all versions of theinvention. Conductive traces 38 are connected to at least one of thecircuits 45, (connection not shown). While only partially illustrated inFIG. 2A, it should be understood that the conductive traces 38associated with each column of electrodes 36 are disposed over the leg41 that extends rearwardly from adjacent that column of electrodes.

Antenna 44 receives signals that both power the electrodes 36 and thatcontain instructions indicating through which electrodes 36 current isto be pulsed. Circuits 45 are configured to: harvest the power in thereceived signals; demodulate the signals to obtain the instructionscontained in the signals; and establish the electrode to power supplyconnections specified by the instructions. A more detailed descriptionof this type of assembly is disclosed in Applicants' Assignee's PCTPatent Application, Implantable Neuromodulation System IncludingWirelessly Connected Pulse Generator And Electrode Array Assembly, PCTApp. No. PCT/US2007/088580, filed 21 Dec. 2007, PCT Publication No. WO2008/080073, the contents of which are incorporated herein by reference.In general though it should be understood that disposed on the assembly28 is a circuit for storing the power of the received signals. A switchcircuit selectively connects the individual electrodes to the anode andcathode sides of the power supply and storage circuit. Another circuitdemodulates the received signals to extract the instructions. Acontroller asserts control signals to the switch circuit to cause it to,based on the received instructions, establish the appropriate powersupply-to-electrode connections.

In some versions of the invention, circuits 45 are able to monitor theperformance of the assembly. In these versions of the invention, antenna44 is able to transmit signals back to the control module or programmingunit.

Terminals 390, seen in FIG. 36 and described with respect to electrodearray 290 of FIG. 35, can be used to connect conductive traces 38 tocircuit components 45.

Electrode array assembly 28 is further formed to have a number ofthrough slots 54 in the substrate 34 (one slot 54 identified in FIG.2A). Each slot 54 is associated with and partially surrounds a separateone of the electrodes 36 in the first, second, third and fourth columnsof electrodes. Slots do not surround the electrodes in the fifth columnof electrodes of the assembly of FIG. 2A. Each slot 54 has threesections. There are upper and lower sections 56 and 60, respectively,located on the opposed ends of an electrode when viewed in FIG. 2A. Acenter section 58 connects the upper and lower sections 56 and 60 ofeach slot 54. Slot center sections 58 are located immediately to theside of the associate electrodes 36 opposite the sides to whichconductive fans 39 extend. Each slot 54 defines a tab 55 out of asection of the electrode array assembly. In FIG. 2A the section ofassembly 28 between adjacent slot upper and lower sections 56 and 60,the section of the assembly between longitudinally adjacent tabs 55, isidentified as a beam 53. An electrode 36 is located on each one of thetabs 55. In the depicted version of the invention, the electrodes 36 inthe fifth column of electrodes, the rightmost column in FIG. 2A, are notdisposed over tabs.

Electrode array assembly 28 is further formed to have a number ofsupplemental slots 61. Each supplemental slot 61 generally has threesections shaped to define an auxiliary tab 62. Each auxiliary tab 62 islongitudinally aligned with the electrodes in one of the columns ofslot-bordered electrodes 36. Slots 61 are located so that auxiliary tabs62 are located immediately rearward the top curved face of the assemblyhead 40.

Also formed in the assembly head 40 are two circular alignment openings63. As described below, alignment openings 63 facilitate the manufactureof the assembly 28.

Substrate 34 of the electrode array assembly 28, seen in FIGS. 3 and 4,is formed from a polyxylene polymer film, such as parylene-C which isavailable from Specialty Coating Systems, Inc. In some versions of theinvention, substrate 34 typically has a thickness of at least 1 micron.In a number of versions of the invention, substrate 34 has a thicknessbetween 5 and 10 microns.

Each conductive trace 38 and associated conductive fan 39 are formedfrom multiple layers of conductive metal. A bottom layer 64 of eachconductive trace, as well the associated fan 39 is formed from chrome.Bottom layer 64 typically has a thickness of less than 280 Angstroms.Bottom trace 64 is provided because deposited chrome bonds to bothpolyxylene polymer and gold. Gold is the material from which the traceintermediate layer 66 is formed. Typically, intermediate layer 66 has athickness of 5 microns or less. In more preferred versions theintermediate layer has a thickness of approximately 2 microns. Theintermediate layers 66, being formed of gold, function as the lowresistance conductive components of the conductive traces 38 andconductive fans 39. A top layer 68 is disposed over intermediate layer66 and forms the topmost layer of a trace 38 and associated fan 39. Toplayer 68, like bottom layer 64 is formed from chrome and has a similarthickness as the bottom layer 64. Top layer 68 is provided because thechrome of this layer, like the chrome of bottom layer 64 bonds well tothe other materials from which the assembly of this invention is formed.

A non-conductive outer shell 72 is disposed over the surfaces of thesubstrate that support the conductive traces 38 and conductive fans 39.In one version of the invention, shell 72 consists of a second layer ofpolyxylene polymer parylene-C film. Shell 72 typically has a thicknessof at least 1 micron. In a number of versions of the invention, shell 72has a thickness of between 5 to 10 microns. Shell 72 insulates andprotects the conductive traces 38 and conductive fans 39 and also coversthe surfaces of substrate 34 that are trace, conductive fan andelectrode free. As discussed below, sections of the shell 72 also coverportions of the electrodes 36.

Each electrode 36 may include a conductive base pad 76 from which anumber of conductive buttons 86 project. In one version of theinvention, base pad 76 includes the same three layers of material thatcomprise the conductive traces 38 and conductive fans 39. There is achrome bottom layer 78; a gold intermediate layer 80 and chrome toplayer 82. As discussed below, the layers forming the electrode base pad76 are formed simultaneously with the layers forming the conductivetraces 38 and conductive fans 39. Accordingly, base pad layers 78, 80and 82 have the same thickness as trace/branch layers 64, 66 and 68,respectively.

Each conductive button 86 typically has a circular cross sectionalprofile when viewed from above. The diameter of the button is typicallyat least 10 microns. In many versions of the invention, thecross-sectional diameter of the button 86 is between and 20 and 40microns. Conductive buttons 86 are typically not visible to the eyewithout magnification. For purposes of illustration only, dots representthe buttons 86 on one column of electrodes in the assemblies depicted inFIGS. 2 and 2A. This is for illustration only. Throughout the rest ofthe drawings, conductive buttons 86 are depicted as much larger in sizethan they are in actuality. This is to make the buttons 86 visible forpurposes of illustration.

Each button 86 has a pedestal 88 that is disposed directly over theadjacent base pad top layer 82. Each pedestal 88 is formed from titaniumand has a thickness of at least 100 Angstroms. In many versions of theinvention, pedestal 88 has a thickness of around 300 Angstroms. A head90 formed from iridium is disposed above each pedestal 88 and is thetopmost component of the button 86. The head 90 has a thickness of atleast 100 Angstroms and is more often at least 1000 Angstroms thick. Insome versions of the invention button head 90 has a thickness of around1500 Angstroms.

Non-conductive shell 72 is also disposed over the electrodes 36. Moreparticularly the film forming the shell 72 is disposed over the outersurfaces of the base pad top layer 82 that are button-free. Shell 72also extends around the outer perimeter of the exposed faces of thebutton heads 90. Openings 92 in the shell 72 expose the faces of thebutton heads 90 inward from their outer perimeters.

Frame 32 is formed from a superelastic material, that is, a materialthat, once formed, returns to the formed shape after being subjected toappreciable deformation. In one version of the invention frame 32 ismetal, more particularly a nickel-titanium alloy. One such alloy isNitinol. Frame 32 has a minimal thickness of 10 microns. In manyversions of the invention, frame 32 has a thickness of at least 10microns. In some versions of the invention, frame 32 has a thickness ofat least 25 microns. When electrode array assembly 28 is curved, thecurvature of the frame along its lateral axis, the axis that is curved,can be as little as 0.5 cm. In many constructions of the assembly 28,the frame has a curvature of between 3 to 5 cm. Larger curvatures arealso possible.

II. Method of Assembly

The process steps by which the electrodes 36, conductive traces 38 andconductive fans 39 of assembly 28 are formed are now discussed byinitial reference to FIG. 5. Initially a silicon oxide layer 122 havingthickness of approximately 2 microns is formed over the exposed surfaceof a silicon wafer 120. While not further described, it should beunderstand that after each step of depositing a layer of material thatforms the electrode array assembly 28, the thickness of the layer ofmaterial is verified and the exposed surface is cleaned. As is discussedbelow, silicon oxide layer 122 functions as a sacrificial layer for thepartially fabricated assembly 28.

A layer 124 of parylene-C is deposited over the silicon oxide layer 122,FIG. 6. In one method of fabricating assembly 28, parylene-C layer 124is applied by vapor deposition. Paralyne-C layer 124 as discussed below,eventually becomes part of the substrate 34 over the frame 32. Duringthe subsequent manufacturing steps in which the electrodes 36,conductive traces 38 and conductive fans 39 are formed, parylene-C layer124 functions as the substrate on which these components are formed.

A thin layer of chrome, layer 126 in FIG. 7 is then disposed overparylene-C layer 124. Chrome layer 126 subsequently forms both theconductive trace and conductive fan bottom layers 64 and the electrodebase pad bottom layers 78. Immediately after chrome layer 126 is appliedto parylene-C layer 124, a small layer 128 of gold is applied over thechrome layer 126. Gold layer 128 has a thickness of approximately 500Angstroms. Gold layer 128 functions as a seed layer of gold for thesubsequent layer 134 that forms the primary conductive components of theassembly 28. Chrome layer 126 and gold layer 128 are applied via anevaporation or a sputtering process.

Once gold layer 128 is applied to the wafer, a photo resist mask 132 isapplied to the surfaces that do not support electrodes 36, conductivetraces 38 or conductive fans 39. FIG. 8 represents the application ofmask 132 along a cross sectional slice of the wafer 120 at which anelectrode 36 is being fabricated. Mask 132 should have a height oftypically more than 3 microns and more often approximately 4 microns.

Once mask 132 is applied, gold, identified as layer 134, is applied overthe exposed surfaces of gold layer 128, FIG. 9. Since the materialforming layers 128 and 134 are identical in FIG. 9, and the subsequentFigures, where these layers overlap only layer 134 is called out. Anelectroplating process is used to apply gold layer 134 so that it hasthe thickness necessary to function as both the intermediate layers 66of the conductive traces 38 and conductive fan 39 and the intermediatelayers 80 of the electrode base pads 76.

In the electroplating process in which gold is applied to the wafer 120,the potential is applied to gold layer 128 that extends over the wholeof the workpiece. This ensures that the gold forming layer 134 uniformlyadheres to all of the exposed and spaced apart mask free surfaces of theworkpiece where intermediate layers 66 and 80 are to be formed.

Once gold layer 134 is applied to the workpiece, a layer of chrome 136is applied to the exposed surface of gold layer 134. Chrome layer 136becomes the top layers 68 of the conductive traces 38 and conductivefans 39 and the top layers 80 of the electrode base pads 76. Chromelayer 136 is applied by evaporation or sputtering.

While not seen in the Figures, it should be understood that some of thegold and chrome released to form, respectively, layer 134 and layer 136coats the outer surface of mask 132.

After chrome layer 136 is applied, mask 132 is removed, (post-removalview not shown.) As represented by FIG. 10, a photo resist mask 138 isthen applied over chrome layer 136, as well as the underlying gold layer134 and chrome layer 126. Mask 138 is also applied to the sides of goldlayer 134 and chrome layer 136.

Once mask 138 is applied, the unprotected portions of gold layer 128 aswell as the portions of chrome layer 126 that underlie the unprotectedportions of gold layer 128 are removed. In some versions of the methodof fabricating assembly 28 this is performed by a wet etch process. As aconsequence of the removal of the unprotected portions of layers 126 and128, as depicted in FIG. 11, what remains on the parylene-C layer 124 isthe protected portions of chrome layer 126, the overlying gold layer 134and the top most chrome layer 136. These chrome-gold-chrome sets oflayers become the conductive traces 38, the conductive fans 39 and theelectrode base pads 76. These chrome-gold-chrome sets of layers alsoform the below-described bond pads (not illustrated).

As mentioned above mask 138 covers the sides of gold layer 134. This isdone to avoid undercutting the layered chrome/gold/chrome. Therefore, itmay be necessary to apply a relatively thick layer of mask adjacentthese side surfaces to minimize the likelihood of such undercutsdeveloping. Accordingly, a very small etch protect step comprising asmall tail of gold layer 128 and underlying gold layer 126 does remainafter the removal of mask 138. Given the relatively small size of thisstep, it is not illustrated in FIG. 11 and the subsequent drawings.

Fabrication of electrode array assembly 28 continues with the forming ofthe conductive buttons 86 of the electrodes 36. The fabrication ofbuttons 86 starts with the application of a photo resist mask 146, shownin FIG. 12, over substantially the whole of the workpiece. Mask 146 hasa thickness greater than that of the conductive buttons 86 that areformed subsequent to the application of the mask 146. While mask 146covers most of the wafer 120, there are openings 148 in the mask 146over the exposed surfaces of the sections of chrome layer 136 that formthe electrode bond pads 76.

Once mask 146 is formed, first titanium and then iridium are applied tothe workpiece so these metals are deposited into the mask openings 148,FIG. 13. The titanium is deposited onto the exposed sections of chromelayer 136 so as to form the button pedestals 88. Once the titanium isdeposited, iridium is deposited in mask openings 148 to form buttonheads 90. In some methods of this invention, the titanium and iridiumare deposited in separate sputtering steps. Not shown in FIG. 13 are thetitanium and iridium deposited over mask 146.

Once the electrode buttons 86 are formed, mask 146 and the metalscovering the mask are removed, (post removal view not shown). Parylene-Cis then deposited over the whole of the wafer, including the conductivebuttons 86 as seen in FIG. 14. This layer of parylene-C forms assemblyouter shell 72. The parylene-C forming outer shell 72 is also disposedover the exposed sections of the chrome that forms the electrode basepad top layer 82 over which the buttons are not present. The parylene-Calso covers the exposed sections of the wafer over which the electrodearray assembly or plural assemblies are not being fabricated.

Once the outer shell 72 is formed on the wafer, portions of the shellare selectively etched to define openings 92 over electrode buttons 86.In one method of assembly of this invention, this process starts withthe application of a photo resist mask over the outer shell 72, (masknot illustrated). This mask is formed to define openings. Each openingis centered over a separate electrode button 86. More particularly, theopenings are formed such that each opening in the mask subtends an arealess than the face area of the button 86 with which the opening isassociated.

The parylene-C forming the outer shell 72 is then selectively removed.In one version of the invention, an oxygen plasma is employed toselectively remove the exposed sections of parylene. As a consequence,as seen in FIG. 15, outer shell 72 is perforated with the openings 92through which the iridium heads 90 of the electrode buttons 86 areexposed. Often the electrode array assembly 28 is fabricated so thateach opening 92 has an outer diameter that is at least 2 microns lessthan the diameter of the associated conductive button 86. In somepreferred versions of the invention, each opening 92 has a diameter thatis at least 5 microns less than the diameter of the associatedconductive button 86.

Thus, at the end of this process step, the parylene-C of the outer shell72 is present both around and over the top perimeters of the conductivebuttons 86. Outer shell 72, in addition to insulating the conductivecomponents of assembly 28, also holds the conductive buttons 86 ofelectrodes 36 in position.

Also, while not illustrated, the outer chrome layer of the layersforming bond pads (not illustrated) may be removed. This may be done forthe ease of later bonding either wires or contact pads of components 45to the bond pads.

The wafer 120 on which the partially assembled electrode array assemblyis formed is then processed to define slots 54 and 61 and openings 63.Also, excess parylene-C is removed from wafer 120 in this sequence ofsteps to define the perimeter of the assembly. While not illustrated,small dots that form perforations may also be formed in the assembly.These dots allow for the flow through of wet etchant to the sacrificialsilicon oxide layer 122 in the below-discussed release process.Specifically, a photo resist is deposited over the whole of theassembly, the outer shell 72 and the exposed surfaces of the electrodebuttons 86. The photo resist layer is then selectively removed in thelocations where the slots 54 and other openings are formed. An oxygenplasma process is then used to remove both the exposed portions of theouter shell 74 as well as the underling layer 124 of parylene-C. Thisprocess removes material down to, but not including, the silicon oxidelayer 122. As seen in FIG. 16, this process results in the removal ofthe parylene sufficient to form openings such as the slots, (slotsections 56 and 60 illustrated.)

While not illustrated, it should be understood that this step is used toremove the parylene surrounding the substrate sub-assembly. Presentlyapproximately 25 substrate sub-assemblies can be formed simultaneouslyon a single 100 mm diameter wafer 120.

The partially assembled array, the components on the bottom layer ofparylene-C, now substrate 34, is then removed from wafer 120.Specifically, a wet etch process is used to remove the sub-assembly fromthe silicon oxide layer 122. This leaves the electrodes 36, conductors38 and conductive traces 39 disposed above parylene-C 124 as seen inFIG. 17. Alternative release methods could be employed. One such processis a xenon etch process.

FIG. 18 is a flow chart of the initial process steps used to form theassembly frame 32. Initially, in step 162, the physical features of theframe are formed. Thus in step 162 the frame is provided its shape, thetop, bottom and side edges. Also in step 162 openings 164, seen in FIG.19, that become part of the assembly slots 54 are formed. Also formed instep 162 are frame alignment openings 166. These openings 166 becomesections of the assembly alignment openings 63. While not called out inFIG. 19, also formed in the frame in step 162 are the openings that formsections of slots 61 and the supplemental openings 65 (FIG. 2A) adjacentslots 61. Frame 32 may be so formed by an etching process,micromachining or a laser cutting process.

In a step 170, frame 32 is heat set to have the desired curvature.

Once the frame is formed and curved, the substrate-electrodesub-assembly is bonded to it. This process starts with the applicationof a layer 180 of parylene-C to the outer surfaces of the frame. Thisparticular layer of parylene has a thickness of approximately 5 microns.As depicted in FIG. 20, at a minimum, parylene-C layer is applied to theinner curved surface of the frame 32, the surface to which theelectrodes 36 and associated components are subsequently bonded. In someversions of the invention, one or more sub-steps are used to applyparylene-C layer to the inner and outer curved surfaces and exposededges of the frame 32.

A vapor deposition process is used to coat the parylene layer over theframe 32. In this process, vaporized parylene conforms to the surface ofthe frame. Accordingly, the parylene-C does not form a thin film overopenings 164 and the other openings formed in the frame. (In the crosssectional view of FIG. 20, only frame openings 164 are shown.)

The substrate assembly and frame 32 are then placed in registration forthe bonding process. In one method of practicing this step, thesubstrate-and-electrode assembly is inverted, so that the bottomparylene-C layer is the topmost layer. The subassembly is placed on analignment jig 182 seen in FIG. 21. More specifically, the subassembly isplaced on a lower frame plate 184 previously positioned over the topsurface of the alignment plate 184. Lower frame plate 184 is formed fromaluminum and has a thickness of approximately 12 mm.

A pair of alignment pins 186 extend upwardly from the surface of thealignment jig 182. Pins 186 are removably seated in bores 188 (oneshown) in the alignment jig. The lower frame plate 184 is provided withthrough holes 190 (one shown) that allow the plate to be seated overpins 186. The substrate-and-electrode sub assembly is seated on thelower frame plate 184 so that pins 186 extend through the openings inthe substrate that form sections of the alignment openings 63.

Frame 32 is then positioned over the substrate-and-electrode assembly sothat the inner curved surface of the frame faces the exposed parylene-Clayer 124. More particularly, in this step the frame is fitted to thealignment jig 182 so that the pins 186 extend through the framealignment openings 166. Due to the positioning of the alignmentopenings, when the frame 32 is so positioned, the frame bends from itscurved shape into a planar shape. Also, the electrodes 36 disposed overtabs 55 go into registration over the tabs with which they areassociated. It should be understood that, at this time, parylene-C layer180 integral with frame 32 abuts parylene-C layer 124 of thesubstrate-and-electrode assembly. Dashed line 187 represents the borderbetween these two layers 124 and 180. Not identified is the separationbetween the parylene-C forming layer 124 and the parylene-C formingshell 72.

Also not seen in FIG. 21 is the section of parylene-C layer 180 on theside of frame 32 opposite the side on which the electrodes 36 areformed. Post manufacture, this parylene-C layer 180 can be considered tobe insulating layer 37 on the underside of frame 32 as seen in FIGS. 3and 4.

An upper frame plate 192 is then disposed over the outer surface of theouter surface of the frame 32. Frame plate 192 is formed from a rigid,thermally conductive material such as aluminum. Openings 194 (one shown)are formed in frame plate 192. When the upper carrier plate 192 isseated on the alignment jig 182, pins 186 align to plate openings 192.

The lower carrier plate-substrate and electrodes-carrier-upper carrierplate assembly is then placed in a press unit 202, FIG. 22. During thetransport process, pins 186 are part of this assembly and are used tohold the substrate-and-electrode assembly and the frame 32 inregistration.

Press unit 202 includes a static platen 204 and an opposed moving platen206. Closed end bores 188 extend from the exposed face of the staticplaten. Moving platen 206 is provided with through holes 205, (oneshown) for receiving pins 186. Threaded drive rods 207 represent themechanism integral with press unit 202 that move the moving platen 204towards and away from the static platen. For reasons that are apparentbelow, the press unit 202 is disposed in a vacuum chamber.

The assembly holding the substrate and frame is positioned so that theexposed surface of the lower carrier plate 184 is seated against thestatic plate and the exposed surface of the upper carrier plate 192 isadjacent the moving plate 206. After the assembly is so positioned, themoving plate 206 is urged against the assembly so that a sufficientforce is exerted to eliminate lateral slippage of thesubstrate-and-electrode sub-assembly relative to the frame 32. Pins 186may then be removed from the press.

The actual bonding process is now described by reference to the timingdiagram of FIG. 23. Initially, the chamber housing press unit 202 withthe stacked substrate and electrodes-frame is closed, time t₀. At timet₁, a suction is drawn on the chamber to form a vacuum down to at least10 micro torr. In some methods of this invention, the suction is drawnto 5 micro torr or less. The suction is drawn to reduce the content ofoxygen in the chamber. This reduces the likelihood that, in the laterheating step, the parylene-C will oxidize. This oxidation, if allowed tooccur, could place fracture-inducing stresses on the parylene orotherwise degrade the properties of the parylene. Force is then appliedto the moving platen 206 so that this platen urges frame parylene-Clayer 180 against substrate parylene-C layer 124, time t₂. In thisprocess sufficient force is applied against the substrate andelectrodes-frame assembly to create approximately 4-12 MPa of pressurebetween the two adjacent parylene layers.

After the pressure has been applied, at time t₃, the assembly is alsoheated to a temperature slightly above the glass transition temperaturefor the parylene-C but below the melting point. This temperature isbetween 150 and 320° C. This heating is performed by actuating a heatingelement 208 disposed in the static platen 204, time t₃.

It has been found that in one version of practicing this invention, ittakes approximately 15 minutes once heating element 208 is actuated, forthe temperature of the parylene-C layers 124 and 180 to rise to abovethe parylene-C glass transition temperature, the transition from time t₃to time t₄. The actual time varies with the equipment used to performthe process. Once the parylene-C layers are in this state, the pressurecauses these layers to form a uniform bond along the surfaces where theyabut. As a consequence of the bonding of these parylene-C layers 124 and180, the layers form the substrate 34 of the assembly 28. It has beenfound that, for a continuous bond between the parylene layers to formthat can withstand shear without delaminating, the heat should beapplied for approximately 30 minutes, time t₅. At this time, heatingelement 208 is deactivated.

As a consequence of the deactivation of heating element 208, theelectrode array assembly cools to ambient temperature. The rate at whichthe assembly cools is regulated. This rate may be controlled by cyclingthe heating element (step not shown.) Alternatively, the insulationsurrounding the enclosure in which the fixture is located is selected tohave a thermal conductivity that limits the rate at which heat leavesthe fixture.

The assembly is cooled at a relatively slow rate due to its geometry.Specifically, owing to the presence of the metal forming the electrodes36, the conductive traces 38, and the conductive fans 39, during thestep of applying pressure, the top platen 206 is not in contact with thewhole of the top surface of the assembly. Thus, when the pressure isapplied to the parylene-C, it is applied unevenly. This unevenapplication of pressure subjects the surface of the parylene to unevenstresses. After the heating of the assembly is terminated, the metalforming the electrodes 36, the conductive traces 38 and the conductivefans 39 cools more rapidly than the adjacent parylene. This differentialcooling causes differential contraction of the elements of the assembly.Differential contraction causes mechanical stresses at the interfacesbetween these elements. To compensate for these differentialcontractions, the assembly is allowed to slowly cool. The slow coolingallow the parylene to slowly creep. These mechanical stresses can bemitigated, in part, if the rate of cooling is slowed to allow theparylene to creep. The creeping of the parylene results in therelaxation of these stresses.

When the temperature drops to 140° C. or below, time t₆, the pressureplaced on the assembly 28 is backed off. The simultaneous release ofpressure on the assembly while the parylene is allowed to creep furtherreduces the total stress on the assembly. This reduction in stressreduces the fracturing of the parylene that could otherwise occur.

After the pressure on the assembly 28 is backed off, the assemblyremains under vacuum as its temperature continues to drop. When thetemperature drops to approximately 80° C., time t₇, the assembly isready for removal from the chamber in which it is fabricated.Immediately prior to this removal, the suction draw is terminated.

Once electrode array assembly 28 is fabricated, other components such asantenna 44 and circuits 45 may be added to the assembly. Electrode arrayassembly 28 is then folded. In one method of this invention, theelectrode array assembly is disposed in a folding loom 220, FIG. 24,that includes seven spaced parallel apart wires 211-216. Assembly 28 issandwiched between lower wires 211, 212 and 213 and upper wires 214,215, and 216. As a consequence of the placement of the electrode arrayassembly 28 in the folding loom 210, the assembly flexes outwardly fromits curved shape to a planar shape.

As will become apparent below, the diameter of wires 211-216 establishesthe radii of the folds of the electrode array assembly 28. The minimumradii of these folds need to be above the radius at which such foldingimposes strains above the elastic limit for the material being folded.The critical material for some versions of this invention is the Nitinolforming the frame 32. Nitinol typically has an elastic limit of 10%strain. For an assembly having a Nitinol frame of 50 microns thickness,wires 211-217 should therefore have a minimum radius of 0.3 mm.

Also, the metals forming the electrodes 36, the conductive traces 38 andconductive fans 39 may be difficult to fold without being subjected tofracture inducing stresses. Accordingly, it is preferred that theassembly be fabricated so that the metals forming the electrodes 36, theconductive traces 38 and conductive fans 39 not be located over regionsof the assembly that are subjected to the now being described foldingprocess.

In addition to being sandwiched between the sets of wires 211-213 and214-216, the electrode array assembly 28 is aligned so each column ofelectrodes 36 is generally positioned over or under one of the loomwires. Thus, loom wires 211, 212 and 213 are, respectively, disposedbelow the first, third and fifth columns of electrodes 36 of theassembly of FIG. 2. Loom wire 214 is disposed over the surface of theassembly opposite the surface against which wires 211-213 are disposedbetween loom wire 211 and the adjacent side edge of the assembly. Loomwires 215 and 216 are disposed on the same surface of the assembly overwhich loom wire 214 is disposed. Loom wires 215 and 216 are disposed,respectively, over the second and fourth columns of electrodes 36. Whilenot required, in some methods of practicing this invention, theelectrode array assembly 28 is positioned relative to the folding loom210 so that the longitudinal axis of each column of electrodes isapproximately, if not precisely, aligned with the associated loom wire211, 212, 213, 215 or 216.

Once the electrode array assembly is positioned in the folding loom 210,the loom is actuated. In this process the loom wires 211-216 aresimultaneously moved so as to fold the electrode assembly. Specifically,loom wire 212, the loom wire below the assembly 28 associated with thethird column of electrodes is moved upwardly, represented by straightarrow 230. Wires 215 and 216 are moved in downwardly symmetric arcuatepaths, represented by, respectively, arrows 231 and 232.

The wires associated with the first and fifth columns of electrodes,respectively wires 211 and 213, are moved in symmetric paths upwardlytowards the plane in which wire 224 moves, represented by, respectivelycurved arrows 233 and 234. Wires 214 the wires immediately above andinward the left side outer perimeter of the electrode array assemblymoves downwardly and inwardly toward the plane along which wire 212travels as represented by curved arrow 235.

Collectively, the displacement of wire 211 and, on the other side of theassembly, wires 214 and 215 bend the assembly to form the first foldrepresented by dashed line 262 in FIG. 2. The displacement of wire 215,and on the other side of the assembly wires 211 and 212, bend theassembly to form the second fold represented by dashed line 264. Thedisplacement of wire 212 and on the other side of the assembly 28, wires215 and 216, bends the assembly to form the third fold, represented bydashed line 266. The displacement of wire 216 and, on the other side ofthe assembly 28, wires 212 and 213, bends the assembly to establish thefourth fold, represented by dashed line 268.

The presence of slots 54 and 61 selectively weaken the electrode arrayassembly along specific longitudinal axes. These are the axes with whichthe wires 211, 121, 215 and 216 are aligned when the assembly is placedon the loom 210. Accordingly, the material on the assembly along theseaxes essentially become folding axes along which the electrode arrayassembly is folded.

The actuation of folding loom 210 does not result in the whole of theelectrode array assembly bending. The individual electrodes 36 aredisposed on tabs 55 defined by slots 54. Accordingly, while the sectionsof the frame 30, the substrate 34 and outer shell 72 that form beams 53between the tabs 55 bend, the tabs themselves do not bend. Instead eachcolumn of tabs essentially rotates, and to some extent translates aroundthe loom wire 222, 224, 226, 230 or 232 with which the tab is aligned.Consequently, as depicted in the FIG. 25 the electrode-carrying tabs 55and auxiliary tabs 62 do not bend to the extent the rest of the assembly28 bends, is folded. These sections of the assembly substantiallymaintain their planar profile, (discounting the curvature of theassembly 28 as whole.)

Also, it should be understood that the loom 210 is constructed and theelectrode array assembly is positioned so that foot 43 is not folded.

Further, while the exact method is not part of this invention, it shouldbe understood that a part of the construction of electrode array is theformation of antenna 44 on foot 43 and the mounting of components 45 tothe foot.

III. Insertion Tool

As a consequence of the assembly being folded, the assembly can beplaced in the lumen of a cannula or introducer needle for percutaneousinsertions. Thus, it has been found that an electrode array assembly ofthis invention that has width of approximately 7 mm can be folded intofive sections, each of which include a column of electrodes 36 and fitin a cannula having a lumen of less than 3 mm in diameter at widestaxis.

A cannula, identified as deployment or inner cannula 240, used to insertelectrode array assembly 28 against tissue internal to the body isillustrated in FIG. 26. Inner cannula 240 is formed from superelasticmaterial that defines a lumen 242. Cannula 240 is shaped to have anelliptical or oval cross-sectional profile. Lumen 242 has the sameprofile as the body of the inner cannula 240. In FIG. 26 line segment246 represents the major axis of cannula lumen 242, the widest diameterline across the lumen. Electrode array assembly 28 is fitted in thelumen so that the tabs 55 and 62 are in planes that are approximatelyparallel to the plane of major axis of the lumen 242.

To gain access to the epidural space and thereby facilitate fitting theelectrode array assembly 28 against the tissue, the assembly comprisingthe inner cannula 240 with electrode array assembly 28 fitted therein isfitted into an access or outer cannula 250 as seen in FIG. 28. Outercannula 250 has an anti-coring distal end opening. One such design isthe Touhy-style tip. That is, the distal end opening of the cannula isnot axially aligned with the longitudinal axis of the bore down thecenter the cannula. Instead, cannula 250 has an opening 252 that iscentered around an axis that is angularly offset relative to thelongitudinal axis of the lumen of the cannula. In the illustratedversion of the invention these two axes are angularly offset byapproximately 120°. This angle is exemplary, not limiting. The sectionof the distal end of the cannula around the most distal section ofopening 252 is formed to have an outer edge 254 that is relativelysharp. This allows the cannula 250 to be inserted through tissue. Theopposed end of the body of the cannula that defines the proximal end ofopening 252 has a rounded surface 256. The presence of rounded surface256 minimizes the extent to which the electrode array assembly 28, whenbeing deployed is scored.

Proximally rearward of opening 252, outer cannula 250 is shaped to havea body 260, seen in FIG. 27, with an elliptical or oval cross sectionalprofile at least in the section thereof in which the electrode arrayassembly is seated. As seen in FIG. 27 it should further be appreciatedthat the major axis of the lumen 251 of outer cannula body 260, like themajor axis of the lumen 242 of inner cannula 240, is smaller in lengththan the width of the electrode array assembly 28 when the assembly isunfolded. The assembly comprising electrode array assembly 28 and innercannula 240 is positioned in the outer cannula 250 so that, as seen inFIG. 28, in the outer cannula body 260 the major axis of the innercannula is aligned with the major axis of the outer cannula. Thisprocess is performed before the actual insertion of the inner cannulainto the patient.

Once the components are assembled together as described above, thedistal end of the outer cannula 250 is inserted into the body of thepatient. Cannula edge 254 functions as a knife edge that cuts throughthe tissue. Outer cannula 250 is positioned so that opening 252 is abovethe surface against which the electrode array assembly is to bepositioned. In FIG. 29, the outer cannula is shown positioned slicedthrough interspinus ligaments between two vertebras. Outer cannulaopening is in the epidural space above the surface of the dura 31 overwhich the electrode array assembly 28 is to be deployed.

When assembly 28 is in the above position, a pre-deployment position,the orientation of the assembly relative to the surface of the dura isas seen in FIG. 30. Specifically it can be seen that the individualfolds of the assembly 28 are approximately parallel to the surface ofthe dura against which the assembly is to be deployed. If the assemblywhere allowed to be deployed, unfold, in this orientation, the assemblywould expand in the epidural space between the dura and the innersurface of the overlying vertebra. Deployment along this plane would notresult in the electrodes 36 being oriented toward the surface of thedura 31.

Inner cannula 240, with the electrode array assembly 28 containedtherein, is then pushed forward out of outer cannula opening 252 as seenin FIG. 31. To be so discharged, the electrode array assembly28-and-cannula 240 assembly moves along a path of travel that curves inthe transition from the axis of the body outer cannula 250 to the axisof outer cannula opening 252. Owing to the relatively thin profile ofthe electrodes, the conductive traces and the conductive fans and therelatively large radius of the curve along which the electrodes travel,these components are not subjected to significant bending as they travelthis curved path. Since no component is so bent, the possibility thatsuch bending would cause fracture of the components forming theelectrode array assembly is substantially eliminated.

With respect to the described electrode array assembly 28, it isbelieved that the radius of the curve around which it travels should beat least 1 mm.

Once the inner cannula 240, with the electrode array assembly 28contained therein starts to be pushed out of the outer cannula 250, thecontinued ejection of the inner cannula is accompanied by thesimultaneous rotation of the cannula 250. As represented by curved arrow272 of FIG. 32, the inner cannula 240 with the electrode array assembly28 contained therein, is rotated 90°. As a consequence of this rotation,sequentially depicted by FIGS. 30, 32 and 33, it can be seen that foldsof the electrode array assembly 28 are in planes that are generallyperpendicular to the surface of the tissue against which the assembly isto be placed.

To facilitate the proper orientation of the inner cannula 240 during thedeployment process, the cannula may be initially formed in a twistedshape. Specifically, inner cannula 240 may be shaped so that the majoraxis 246 of the cannula lumen 242 rotates around the proximalend-to-distal end longitudinal axis through the lumen. When innercannula 240, with assembly 28 disposed therein is initially fitted inthe outer cannula 250, the inner cannula is constrained in an untwistedshape. Thus, upon advancement of the inner cannula 240 out of the outercannula 250, the inner cannula is free to rotate to the twisted shape.As a consequence of both this twisting of the cannula 240 to its formedshape and the twisting of the cannula 240 by the practitioner, the innercannula 240 assumes the orientation relative to the dura 31 of FIG. 33.Inner cannula 240 and therefore the electrode array assembly 28 are thusproperly aligned for assembly deployment over the dura 31.

Once the electrode array assembly-and-inner cannula assembly is in theposition, the inner cannula is at least partially retracted back intothe outer cannula 250, represented by FIG. 34. While the inner cannulais so retracted, the electrode array assembly 28 is held in its positionrelative to the outer cannula 250. In one version of practicing thisinvention, electrode array assembly is held in place by inserting a roddown the inner cannula lumen 242. The inner cannula 242 is retractedover the rod. The abutment of the foot 43 of the electrode arrayassembly against the rod prevents the retraction of the electrode arrayassembly 28.

Alternatively, guide wires attached to the electrode assembly head 40restrain the assembly 28 from retracting with the inner cannula 240. Aslong as the guide wires are constrained in the inner cannula 240, theguide wires are have some resistance to buckling. This resistance tobuckling allows the wires to serves as members for restraining theelectrode array assembly 28 as the inner cannula 240 is retracted. Oncethe electrode array assembly is deployed, the guide wires can be used tomake adjustments in the position of the assembly 28. During theseposition adjustments, the curved shape of the assembly head 35 allowsthe assembly to be advanced forward. After the assembly 28 ispositioned, the wires are worked free from the assembly and retractedout through the inner and outer cannulae 240 and 250, respectively.

As a consequence of the inner cannula 240 retracting away from theelectrode array assembly 28, the assembly 28 is free from the physicalconstraint of the cannula 240. Owing to the superelasticity of the frame30, the electrode array assembly unfolds, deploys, to its preformedcurved shape.

Again, prior to deployment, the assembly is oriented so that its foldsare generally perpendicular to the surface of the dura 31 against whichit is to be deployed. The frame 32, and therefore the whole of theassembly 28 unfolds along an axis perpendicular to the planes of thefolds. Owing to the orientation of the assembly 28, the assembly'sunfolding axis is parallel to the surface against which the assembly isto be positioned. Due to the pre-formed curved shape of the electrodearray assembly 28, the assembly, when unfolded, thus conforms to thesurface of the dura 31.

In some deployments of the assembly 28, the assembly is laterallysupported by the tissue against which it is disposed. This support holdsthe assembly 28 to the tissue.

Electrode array assembly 28 is then available to apply current toselected sections of tissue below the assembly. Once the assembly ispositioned, the practitioner can, by experimentation, determine throughwhich electrodes 36 the current should flow in order to have the desiredtherapeutic effect. Since the electrodes are in a row by columnarrangement over the tissue, the practitioner has a wide range ofoptions of the sections of tissue through which the current can beapplied. Thus, the current can be applied through the tissue that:located under a single column of electrodes; located under a single rowof electrodes; located under plural rows and/or columns of electrodes;or located between wide spaced apart electrodes. The ability toestablish a current flow path from multiple available current flow pathsincreases the likelihood that the practitioner will be able to establishcurrent flow through the tissue that has both desired therapeuticeffects and minimal side effects.

Electrode array assembly 28 of this invention is designed so that theindividual electrodes 36 are spaced apart a distance that maximizes thelikelihood that adjacent electrodes, though closely packed together,will, when implanted against tissue, be substantially electricallyisolated from each other. This electrical isolation of the electrodesminimizes the likelihood that a significant current will flow betweentwo adjacent electrodes. These unintended current flows, if allowed todevelop, could result in the inefficient delivery of current pulses tothe nerves.

Electrodes 36 are relatively densely packed on the frame 32. Asmentioned above, in some preferred versions of the invention, theelectrodes in some of the columns are laterally offset from theelectrodes in other columns. These features increase the spatialresolution of the current flow paths through the dorsal column nervetissue the practitioner has available to establish. The increase in thisspatial resolution improves the ability of the assembly to pulse currentthrough the nerve tissue that will result in an optimal therapeuticeffect. Further, this increased spatial resolution minimizes the extentto which the current is pulsed through nerves in which such current flowhas either minimal or potentially adverse clinical effect. The reductionin this current flow improves the electrical efficiency of the implantedassembly 28.

While electrodes 36 are closely packed together, the individualelectrodes 36 are spaced apart a distance that maximizes the likelihoodthat adjacent electrodes, when implanted against tissue, will beelectrically isolated. This electrical isolation of the electrodes 36minimizes the likelihood that a significant current will flow betweentwo adjacent electrodes. These unintended current flows, if allowed todevelop, reduce the spatial resolution of the current flow paths.

Another feature of the electrode array assembly 28 of this invention isthat the signal applied to each electrode 36 is applied through theassociated conductive fan 39. The conductive fan 39 provides a lowimpedance path to the electrode 36 along one of the longitudinal edgesof the electrode 36. This low impedance path minimizes variations incharge delivery distribution over the surface of the electrode 36.

Electrode array assembly 28 of this invention is further constructed sothat the gold of the electrodes 28 is not the actual material that formsthe surfaces from which the current is applied to the tissue. Instead,the iridium heads 90 of the conductive buttons 86 define the surfacesthrough which the current is applied. A benefit of this feature of theinvention is that iridium heads form a low impedance interface with thetissue in comparison to the path through a gold interface. This lowimpedance path minimizes the power loss. Further, iridium has arelatively high charge carrying capacity, in comparison to other noblemetals. This high charge carrying capacity serves to minimize thelikelihood that at some current levels the conductive capabilities ofthe electrode will start to decay.

In this version of the invention, the iridium is fabricated as part ofthe electrodes 36 as layers of the conductive buttons 86. By segmentingthe iridium into the small heads, the likelihood that the iridium, whensubjected to the mechanical stresses of folding, rolling, beingdeployed, or post-implantation flexure, will separate from the rest ofthe electrode is appreciably reduced.

Further, the segmenting of the iridium into the conductive buttonsallows the outer shell 72 to hold the iridium to the rest of theassembly.

It also should be appreciated that the material in the epidural space inwhich assembly 28 is deployed has widely varying impedances. In someregions, this material can have a relatively high impedance. Currentdriven through this material can result in a loss of power. In a nearbyregion, the material above the tissue to which the current is to beapplied can have a relatively low impedance. Current driven through thismaterial can cause the current to spread beyond the target tissue. Thiscurrent spreading reduces the spatial resolution of the current pulses.Collectively, the relative proximity of this high impedance and lowimpedance material to each other can make it difficult to establishcurrent flow paths that will result in the desired clinical effect.

Assembly 28 of this invention when deployed unfolds to a curved shape.This curvature causes the assembly to conform to the surface of theunderlying dura 31. This conformance reduces the distance between theconductive buttons 85 and the dura 31. The extent to which the currentis driven through this layer of material with varying impedances istherefore likewise reduced. Accordingly, the adverse effects of thevariable impedance of this material are similarly abated.

Further, the conformance of the electrode array assembly 28 to theunderlying tissue provides the assembly with a relatively high degree ofstability on the tissue. This stability reduces the likelihood that onceimplanted, the assembly will slip or otherwise become dislodged from itsintended deployed position.

IV. Alternative Electrode Array

FIGS. 35 and 36 illustrate an electrode array assembly 290 constructedin accordance with this invention. Electrode array assembly 290 isdesigned to be disposed against a section of tissue. For example,assembly 290 may be curved so as to be disposed over a section of thespinal cord dura 31 (FIG. 1). Assembly 290 includes a number ofindividual electrodes 292. Electrodes 292 are selectively tied to thecurrent sources and current sinks. When the current sources and sinksare actuated, current flows from one or more electrodes 292 tied to thecurrent source or sources through regions of the tissue that underliesthe assembly 290 to the one or more electrodes 292 tied to the currentsink or sinks. A drive module 294 selectively ties the electrodes 292 tothe current sources and current sinks. Drive module 294 is located atthe proximal end of the assembly. (Here, “proximal” means towards theend of the assembly at the bottom of FIG. 35; “distal” means towards theend of the assembly at the top of FIG. 35).

In FIG. 35, assembly 290 is shown active side up so the electrodes 292and drive module 294 can be seen. The “active” side of the assembly 290is the side of the assembly on which the electrodes 292 are located.Opposite the active side, assembly 290 has a “passive” side.

In FIG. 36, drive module 294 is removed from the assembly 290 so theterminal pad 296 at the proximal end of the assembly can be seen.Terminal pad 296 is the substrate of the assembly 290 to which drivemodule 294 is attached. In many versions of the invention, thecomponents forming assembly 290 are dimensioned so that drive module 294extends rearwardly beyond the proximal end of terminal pad 296.

Three parallel, spaced apart bridges 302, 304 and 306 extend distallyforward from terminal pad 296. The outer two bridges, bridges 302 and306, are each formed with a leg, legs 308 and 310, respectively. Legs308 and 310, which are coaxial, extend outwardly from opposed side edgesof terminal pad 296. Bridges 302 and 306 extend perpendicularly forward,distally forward, from legs 308 and 310, respectively. Feet 312 and 314,respectively, connect legs 308 and 310 to terminal pad 296. Each foot312 and 314 has a proximal end edge that tapers distally forward as thefoot extends away from the adjacent side edge of terminal pad 296.Bridge 304, the center located bridge, extends forward from the distalend of terminal pad 296.

Plural tabs 318 extend outwardly from each bridge 302, 304 and 306. Moreparticularly, at a number of spaced apart locations along the length ofeach bridge 302, 304 and 306, two tabs 318 extend outwardly from theopposed sides of the bridge. At least in the version of the inventiondepicted in FIG. 35, the tabs 318 are arranged in diametrically opposedpairs relative to the bridge 302, 304 or 306, from which the individualtabs extend. Electrode array assembly 290 is further constructed so thatat each longitudinal section on bridge 302 tabs 318 extend, tabs 318also extend from the laterally adjacent longitudinal sections of bridges304 and 306. Thus, in the illustrated version of the invention, tabs 318are arranged in rows wherein two tabs extend outwardly from each bridge302, 304 and 306. The rows of tabs 318 are longitudinally spaced apartfrom each other. In some versions of the invention, the separationbetween the distal end of one row of tabs and the proximal end of thedistally adjacent row of tabs is between 1 to 10 mm. In many versions ofthe invention, this separation is between 2 and 6 mm.

Each tab 318 is generally in the form of a rectangle with roundedcorners. Each tab 318 has a length (measurement in the axis parallel tothe longitudinal axis of the assembly of between 0.5 to 5 mm. Often thislength is between 2 and 4 mm. Each tab 318 has a width, (measurementalong the axis perpendicular to the longitudinal axis of the assembly)of 0.25 to 2 mm. In many versions of the invention, this width isbetween 0.5 to 1 mm. An advantage of providing electrodes with thesedimensions is that it both provides a relatively dense package ofelectrodes while eliminating the need to provide conductors so numerousthey must be located on multiple substrates. It should further beunderstood that each tab 318 attached to one bridge 302 or 304 isseparate from the adjacent tab 318 attached to the adjacent bridge 304or 306. The spacing between the adjacent tabs extending from adjacentbridges is typically no more than 500 microns and preferably 100 micronsor less. This small separation between adjacent tabs reduces the amountof tissue that can grow between the tabs 318. If appreciable tissue wereallowed to grow between the tabs 318, this tissue could inhibit laterremoval of the assembly 290.

Bridges 302, 304 and 306 are each shaped so that the width of the bridgebetween two adjacent pairs of tabs 318 is greater than the width of thesame bridge between the distally adjacent next pair of tabs. Thus, thewidth of bridges between the first pair of tabs, the pair closest todrive module 294, and the adjacent pair of tabs is approximately 0.88mm. The width of the bridges between the second and third pairs of tabs,(the pairs second and third closest to drive module 124) isapproximately 0.80 mm. The width of each bridge between the eighth pairof tabs 318 and the adjacent ninth pair of tabs, the distal pair of tabs318, is approximately 0.32 mm.

Beams 320 extend between the bridges 302, 304, and 306. Moreparticularly, each beam 320 extends between adjacent bridges 302 and 304or between adjacent bridges 304 and 306. In the illustrated version ofthe invention, assembly 290 is further constructed so that each beam 320connecting bridges 302 and 304 is collinear with an adjacent beamconnecting bridges 304 and 306. Each beam 320 has a width, (measurementalong the axis parallel to the longitudinal axis of the assembly 290) ofapproximately 0.25 mm.

The electrode array assembly 290 of FIG. 35 is further constructed sothat there is a pair of collinear beams 320 adjacent the proximal anddistal ends of each of the tabs 318 in each row of tabs. Thus, in theillustrated version of the invention there are 18 pair of beams thatconnected the spaced apart bridges 302, 304, and 306 together. While thebeams 320 are adjacent tabs 318, it should be understood that the beamsare spaced apart from the tabs.

Given the spacing between the tabs 318, it should be appreciated thatthe longitudinally adjacent pairs of beams 320 are spaced apart fromeach other along the longitudinal axis of electrode array assembly 290.As discussed below, a flexible membrane 322 is disposed between theseadjacent spaced apart beams 320. In FIG. 36 membranes 322 are shown bysurface shading. Similarly, there are also membranes 324 located on theouter sides of bridges 302 and 306. Each of the membranes 324 extendsbetween each a pair of longitudinally adjacent tabs 318 that extend fromthe outer sides of bridges 302 and 306. It should be appreciated thoughthere are no membranes between the beam the tabs 318 and the adjacentbeams. Instead, around the outside of each tab 318 that extends inwardlyfrom bridges 302 and 306 and from both sides of bridge 314 there is athree-sided slot (not identified).

Electrode array assembly 290 is also formed to have a head 326 and twoshoulders 340. Head 326 is located forward from a small neck 328 thatforms the distal end of center-located bridge 304. Thus, neck 328 islocated forward of the two distal most tabs 318 that extend outwardlyfrom bridge 304. Each of the two distal most beams 320 extend from neck328. Head 326 is located forward of the two distal most beams 320. Head326 has a proximal edge that extends laterally beyond neck 328 on eitherside of the neck. The head 326 has two parallel side edges. At the mostdistal end, head 326 has an outwardly curved distally-directed frontedge.

Each shoulder 340 extends forward from a small land 342 located forwardof the associated outer bridge 302 or 306. Each land 342 is integralwith and extends distally forward from the outer tab 318 integral withthe bridge 302 or 306 with which the land is attached. Lands 342 serveas the terminuses for the beams 320 that extend from neck 328. Eachshoulder 340 is spaced forward and away from the adjacent beam 320.Shoulders 340 are also spaced laterally away from the adjacent sideedges of the head 326. Specifically, the shoulder 340 on the left sideof FIG. 35 is spaced from the adjacent head side edge along a line thatis collinear with the line along which the tabs 318 associated withbridge 302 are spaced from the adjacent tabs 318 associated with bridge304. Similarly, the shoulder 340 on the right side of FIG. 35 is spacedfrom the adjacent head side edge along a line collinear with the linealong which the tabs associated with bridge 304 are spaced from theadjacent tabs 318 associated with bridge 306.

Each shoulder 340 is approximately in the shape of a right angletriangle wherein the 90° corner is located adjacent the bottom of theadjacent side of edge of the head 326. The hypotenuse edge of theshoulder 340 is the outer edge of the shoulder. Each shoulder 340 is,however, further shaped to have a rounded distal end 344.

A beam 346 connects the hypotenuse of each shoulder 340 to the top ofhead 326. Each beam 346 extends from an outer extension of theassociated shoulder forward and inwardly. Thus, each beam 346 is spacedforward from the distal end 344 of the associated shoulder 340 andcurves inwardly over the adjacent side of the front edge of head 326.The inner end of each beam 346 is connected to a small nose 347 thatextends forward from the most forward edge of head 326. Thus, betweeneach shoulder 340 and associated beam 346 there is a small void space(not identified) that generally has the shape of an arrow head.Immediately to the side of nose 347 there is a small curved slot (notidentified) between each beam 346 and the adjacent distal end edge ofthe head 326. This slot is contiguous with the void space between thebeam 346 and the adjacent shoulder 340.

An electrode 292 is disposed on each one of the tabs 318. Pluralconductors 348 are disposed on bridges 302, 304 and 306. Each conductor348 extends to a separate one of the electrodes 292 integral with theassociated bridge 302, 304 or 306. In FIG. 35, due to scale, the set ofconductors on each bridge is seen as a single black line. The thicknessof this line decreases distally along the length of each bridge. Thisdecrease in line thickness represents that, moving distally along eachbridge 302, 304 or 306, the number of conductors present on the bridgedecrease. Conductors 348 are the conductors over which current issourced to or sunk from the electrodes 292. If an electrode 292 does notfunction as a current source or sink, the electrode may function as avoltage probe. When an electrode 292 performs this function, theassociated conductor 348 serves as the conductor over which the sensedvoltage is connected to the monitoring circuit.

Each conductor 348 only extends as distally forward as the electrode 292to which the conductor is connected. Adjacent the associated electrode,each conductor fans out to have a width (the dimension along the lineparallel to the longitudinal axis of the array 290) that issubstantially equal to the length of the electrode 292. Each bridge 302,304 and 306 therefore supports more conductors adjacent its proximal endthan its distal end. This need to support the largest number ofconductors adjacent the proximal ends of the bridges 302, 304 and 306 iswhy these ends of the bridges are wider than their complementary distalends.

Electrode array assembly 290, as seen in FIGS. 37 and 38 has a frame 350formed from the same superelastic material from which frame 30 of thefirst described electrode array assembly of this invention is formed. Insome versions of the invention, frame 350 has a thickness betweenapproximately 25 and 100 microns. Frame 350 is shaped to form the basicgeometric features of the assembly including bridges 302-306, legs 308and 310, feet 312 and 314, tabs 318, beams 320 and 346, head 326, neck328, shoulders 340 and lands 342. However, as discussed below, frame 350does not serve as a substrate for terminal pad 296. Similarly, membranes322 and 324 are formed from material different from which the frame 350is formed.

Frame 350, like frame 30, may be curved early in the processes ofmanufacturing assembly 290. Often the arc of curvature is perpendicularto the longitudinal axis of the assembly 290. In these versions of theinvention, this means that when a manufactured assembly 290 is placed ona flat surface, with the electrodes 292 facing downwardly, center bridge304 is elevated relative to side bridges 302 and 306 as seen in FIG. 39.More particularly, FIG. 39 illustrates the elevation of center bridge304 relative to bridge 306.

It can further be seen from FIG. 39 that when the frame 350 is soshaped, legs 308 and 310 and feet 312 and 314 may not be curved to theextent the rest of the frame is curved. Also, the proximal end of bridge304, the end proximal to the proximal most beams 320 may be bentdownwardly. This further shaping of the frame 350 is to ensure that theterminal pad 296, which extends from the proximal end of bridge 304 andfeet 312 and 314 is a planar structure.

Insulating material, parylene-C, is disposed on the top, bottom and sidesurfaces of the frame 350 (side-located insulating material not shown).This insulating material is disposed over the surfaces of the frame 350.In FIGS. 37 and 38, the insulating material disposed over the surface ofthe frame 350 away from the tissue against which the assembly 290 isemployed, is identified as passive side insulating layer 352. Insulatinglayer 352 has a thickness of approximately 1 to 20 microns. In manyversions of the invention, the insulating material of layer 352 has athickness of approximately 5 to 15 microns. Passive side insulatinglayer 352, in addition to being disposed over the “passive” side offrame 350 is also disposed over the side edges of the frame 350. Thecoating forming the passive side insulating layer 352 also extendsbeyond the side edges of the frame 350.

The insulating layer disposed over the surface of the frame 350 on whichelectrodes 292 and conductors 348 is located is identified as theintermediate insulating layer 354. Intermediate insulating layer 354 hasa thickness in the range of the thickness of passive side insulatinglayer 352. The material forming intermediate insulating layer 354 alsois located in the area bordered by the proximal end of bridge 304 andfeet 312 and 314. In this area, the material forms a passive side layer356 of terminal pad 296, seen in FIG. 40. This passive side insulatinglayer 356 is formed with a number of openings 358, one opening 358 seenin FIG. 40.

During some methods of manufacturing, the material forming the passiveside insulating layer 352 and the intermediate insulating layer 354 areapplied in a single step. These insulating layers 352 and 354 form acontinuous coating around frame 350.

Conductive traces that form base pads 293 for the electrodes 292, andthe whole of the conductors 348 are disposed on the exposed surface ofintermediate insulating layer 354. Typically, each conductive traceincludes a thin layer of titanium 362 applied directly to intermediateinsulating layer 354. Layer 362 typically has a thickness of 100 to 1000Angstroms. A thicker layer of gold 364 is disposed over titanium layer362. The gold layers 364 that form the electrode base pads have athickness of 1 to 15 microns, typically 8 to 12 microns. The gold layers364 that form parts of the conductors 348 have a thickness of 1 to 3microns. A thin outer layer of titanium 366 is disposed over the surfaceof the exposed surface of gold layer 364. Titanium layer 366 has athickness approximately equal to that of titanium layer 362. Titaniumlayers 362 and 366 and gold layer 364 function as the low resistanceconductive assembly of the conductive traces. However, titanium isrelatively brittle. Gold is more ductile. Therefore, to reduce thelikelihood of conductor breakage due to bending, the layers of thetitanium are kept relatively thin. Titanium layer 362 is provided tostrengthen the bond between the gold to the intermediate insulatinglayer 354. Outer titanium layer 366 is provided to strengthen the bondof the below-discussed active side insulating layer 398 to the goldlayer 364. The outer titanium layer 366 is also provided to facilitatethe bonding of the outer metal layers that form the individualelectrodes 292.

Where this titanium/gold/titanium laminate is formed to function as aconductor 348, the laminate has a trace width of approximately 1 to 100microns. Often this trace has a width between 20 to 50 microns. It hasbeen found that a trace having this width offers a low-impedanceconductive path without occupying a significant surface area. It hasbeen found that these traces can be spaced apart as closely as 1 micron.For reasons of manufacture, these traces are often spaced apart at least5 microns.

It should further be appreciated that the titanium/gold/titaniumlaminates that form conductors should have a thickness of less than 5microns. This limits the strain to which the conductors 348 are exposedwhen folded or bent to that which this laminate can withstand withoutbreakage.

The titanium/gold/titanium laminates that form the electrode base pads293 have a thickness of at least 5 microns and typically at least 10microns. The relatively thick nature of these laminates is to make thebase pads 293 radio opaque. It is desirable to have the base pads 293radio opaque so the position of the assembly can be tracked using afluoroscope. Thus, while not illustrated it should be appreciated thatthe thickness of the gold layers 364 that are part of the electrode basepads 293 are often thicker than the gold layers 364 that are part of theconductors 348. The portion of this laminate that defines the base pad293 of an electrode 292, occupies a surface area of at least 1 mm².Typically this base pad occupies a maximum surface area of 10 mm² andtypically 5 mm² or less. It has been found that electrodes with smallersized base pads are difficult to detect with fluoroscopic instrumentsand with the eye that monitors the images produced by these instruments.

In FIGS. 36 and 40 it can be seen that each conductor 348 is furtherformed to extend a short distance over the adjacent terminal pad passiveside insulating layer 356. Each conductor 348 terminates at aring-shaped terminal 390 also formed over the terminal pad passive sideinsulating layer 356. Terminals 390 are formed during the same processsteps in which conductors 348 and electrode bond pads 293 are formed.Specifically, in this part of the process, each titanium layer 362 isformed to define a circle (not identified) having a diameter equal tothat diameter of the associated terminal 390. This circle, seen in crosssection in FIG. 40, is centered over the opening 358 in the underlyingpassive side insulating layer. This titanium circle has a diametergreater than that of the underlying opening 358. A center opening isformed in this titanium circle (opening not identified). Gold, that iscontiguous with the gold of layer 364, fills the opening in theterminal-forming portion of the titanium layer 362. When the titanium isapplied to define titanium layer 366, this titanium is not applied tothe face of the gold that functions as the face of terminal 390.

An opening 392 exists in the portions of the gold layer 364 and titaniumlayer 362 that define each terminal 390. This opening 392 is concentricwith opening 358 in terminal pad passive side layer 356. Opening 392 issmaller in diameter than the coaxial opening internal to the underlyingtitanium layer 362. Opening 392 has an approximate diameter of 45microns. The gold that defines the outer perimeter of the terminalaround opening 392 has an approximate diameter of 90 microns.

An electrode 292 of assembly 290 of this invention, in addition tohaving the titanium/gold/titanium base pad 293, has two additionallayers deposited above the outermost titanium layer 366 of the base pad.As seen in FIG. 37, a second outer layer of titanium 394 is disposedover the outer surface of titanium layer 366. A layer of iridium 396 isdisposed over the outer surface of the titanium layer 394.

The exposed surface of the iridium functions as the tissue-contact faceof the electrode 292.

An insulating layer, again possibly a polyxylene polymer coating, isdisposed over at least a portion of each electrode 292 and over thewhole of the conductors 348. The insulating layer applied overconductors 348 prevents the conductors from functioning as electrodes.This insulating layer is applied over both the electrodes 292 andconductors 348 as a laminate that adds structural strength to assembly290. In the Figures, this insulating layer is identified as active sideinsulating layer 398. An opening 402 is formed in layer 398 that isconcentric with terminal opening 392. Opening 402 is larger in diameterthan terminal opening 392.

The coating forming the active side insulating layer 398 is alsodisposed over the adjacent surface of the terminal pad passive/sidelayer 356. In this section of the assembly, this coating is called outas the terminal pad active side layer 404. This active side layer 404 isdisposed over the titanium layers 366 of the terminals 390. In FIGS. 40and 41 the terminal pad active side layer 404 is shown to curvedownwardly from the active side insulating layer 398. This is becausethe material from which the active side insulating layer 398 and theterminal pad active side layer 404 are formed is a conformal coating.Over most of the terminal pad 296 neither conductors 348 nor terminals390 are present. Accordingly, in these sections of terminal pad 296 freefrom conductive metal, terminal pad active side layer 404 dips below theadjacent active side insulating layer 398.

In some methods of manufacturing the electrode array assembly of thisinvention, the material forming the active side insulating layer 398 andthe terminal pad active side layer 404 is applied to cover the whole ofthe electrodes 292, the conductors 348 and terminals 390. Portions ofthis insulating material are removed to expose small sections of each ofthe iridium layers 396 of the individual electrodes 292. Theseindividual openings 399 may be circular or rectangular in shape. Thus,while the base pad 293 of an individual electrode may have a surfacearea of 2.25 mm² the exposed surfaces of the iridium layer 396, thesurfaces through which current is sourced to or sunk from the adjacenttissue, may collectively have an area of 1.8 mm².

FIG. 41 illustrates how drive module 294 is attached to the othercomponents of the electrode array assembly 290. Drive module 294includes a semiconductor die 408 on which a circuit for connecting theindividual electrodes 292 to current sources and current sinks isfabricated. As discussed below, in some versions of the invention, thecurrent sources and current sinks may also be fabricated on die 408.Bond pads 410 (one illustrated) are formed on the bottom surface of die408 directed towards the terminal pad active side layer 404. A portionof die 408 may also extend over the active side insulating layer 398.

Die 408 is positioned over the exposed face of terminal pad active sidelayer 404. More particularly, the die 408 is positioned over theassembly terminal pad 296 so that each bond pad 410 is in registrationwith its corresponding terminal opening 392. In FIG. 41, a small gapbetween the outer face of the terminal pad active side layer 404 and die408 is exaggerated for purposes of illustration. A rivet 412 extendsthrough the conductive ring 366 to the bond pad. This rivet 412, whichis often formed from a small droplet of liquefied gold, has a head 413that is formed around the face of the gold layer 364 of the terminalexposed through intermediate insulating layer opening 358. Rivet 412 hasa shaft 414 that extends from the rivet head 413 to the associated diebond pad 410. Each rivet 412 thus connects the associated die bond pad410 to the associated conductive ring 366.

Rivets 412 also hold the die 408 to the assembly terminal pad 296. Insome versions of the invention, additional rivets that do not providedie-conductor electrical connections provide additional mechanicalconnections between the terminal pad 296 and the die 408. It shouldfurther be understood that since terminal pad 296 and feet 312 and 314are planar, die 408 lies flat against the terminal pad. This flatsurface-to-flat surface interface facilitates the bonding of the die 408to the rest of the assembly 290.

An inner capsule 415 extends around the exposed faces of die 408. Innercapsule 415 is thus disposed over terminal pad active side layer 404.The inner capsule 415 is formed from material such as epoxy. In someversions of the invention, die 408 is encased in inner capsule 415before the die is attached to terminal pad 296. The sides of innercapsule 415 extend to the surfaces of the terminal pad active side layer404 that surround the die 408.

Drive module 294 also includes an outer capsule 416 formed from amechanically robust, insulative, biocompatible material, such as epoxyor silicon oxide. Outer capsule 416 includes a shell and a cap 418 and420, respectively. Outer capsule shell 418 covers inner capsule 415 andsemiconductor die 408 encased therein. The side walls of outer capsuleshell 418 abut active side insulating layer 398. More particularly,outer capsule shell 418 is dimensioned to abut the portion of activeside insulating layer 398 that extends over the portion of frame 350that define the proximal end of bridge 304 and feet 312 and 314.

Outer capsule cap 420 is disposed over the terminal pad passive sidelayer 356. Cap 420 also extends beyond the perimeter of passive sideinsulating layer 356. Cap 420 thus extends at least partially over theportions of outer capsule shell 418 that likewise extend beyond theperimeters of the terminal pad passive and active side layers 356 and404, respectively. The adjacent abutting portions of shell 418 and cap420 are sealed or otherwise bonded together to form the outer capsule416.

In some versions of the invention, the proximal end of bridge 304 andassembly feet 312 and 314 are formed with through openings, notillustrated. These through openings extend through insulating layers352, 354 and 404 and frame 350. The material forming either outercapsule shell 418 and/or cap 420 is molded in place. A portion of thematerial forming this molded structure extends through these openings inthe bridge 304 and feet 312 and 314. Upon hardening, this material formsposts that extend through bridge 304 and feet 312 and 314 that furthersecure the outer capsule 416 to the rest of assembly 290.

The sub-circuits that may be formed on the die 408 integral with drivemodule 294 may be similar to the sub-circuits contained within circuits45 of electrode array assembly 40. Electrode array assembly 290 asillustrated does not include an antenna. Power that forms the currentsourced and sunk between the electrodes 292 as well as power to energizethe components internal to the drive module comes from an implantabledevice controller (IDC) 428 (FIG. 35). The IDC 428 is connected to andseparate from the electrode array assembly 290. This connection is madeby a cable 427. In versions of the invention wherein this cable 427 ispresent, wires internal to the cable are connected to bond pads integralwith die 408. These bond pads are separate from and spaced away from thebond pads 410 to which rivets 412 are bonded. In some versions of theinvention, bond wires, not shown are connected from the die bond pads toterminal pads on the adjacent surface of inner capsule 415. The wiresintegral with cable 427 are also connected to the bond pads on the innercapsule 415. The adjacent end of the cable 427 is held to assembly 290by potting between outer capsule shell and cap 418 and 420,respectively.

As seen with respect to FIG. 42, in some versions of the, drive module294 includes a power supply 425. The power supply module may include apower harvesting circuit (not illustrated). The power harvesting circuitstores the power contained in the instruction signals forwards to theelectrode array assembly 290 by the IDC 428. This power is stored in acapacitor or a rechargeable cell, part of the power supply 425 and notillustrated. A constant voltage circuit, also part of the power supply425, outputs one or more constant voltage signals to the othercomponents of drive module 294.

Drive module 294 also includes a number of variable-output currentsources 430 and variable output current sinks 432. (For ease ofillustration, power supply 425 is shown connected to only a singlecurrent source 430. It is understood that the power supply 425 isconnected to each source 430 and sink 432.) In FIG. 42, only two currentsources 430 and two current sinks 432 are illustrated. In practice,power supply module 294 may have three or more current sources 430 andcurrent sinks 432. In some versions of the invention, the number ofcurrent sources 430 and current sinks 432 may each equal the number ofelectrodes 292.

In versions of the invention wherein the number of current sources 430and current sinks 432 is each less than the total number of electrodes,the sources and sinks are connected to the array electrodes 292 througha current multiplexer 434, also fabricated as part of die 408. Currentmultiplexer 434 connects the current sources and sinks 430 and 432,respectively, to the electrodes 292. For ease of illustration in FIG. 42only six (6) electrodes 292, the number of electrodes in a single row ofthe assembly 290 of FIG. 35, are illustrated.

Drive module 294 also includes a control processor 436. Processor 436regulates the magnitude of the current sourced and sunk by,respectively, each source 430 and sink 432. The processor 436 alsoasserts control signals to current multiplexer 434. Based on the signalsasserted by processor 436, multiplexer 434 connects each source 430 andsink 432 to the appropriate electrode/electrodes 292.

It should be understood that, at any given moment, multiplexer 434 isable to connect each electrode to a current source 430 or to a currentsink 432. Nevertheless, it should be appreciated that the switchesforming multiplexer 434 each have at least one failsafe sub switch (notillustrated). Each multiplexer sub switch prevents a single electrode292 from simultaneously being tied to both a current source 430 and acurrent sink 432. This arrangement substantially eliminates thepossibility that a short circuit can develop between a source 430 and asink 432.

In FIG. 42, power supply 425 is shown connected to control processor436. This represents that power supply 425 sources the current thatenergizes the components internal to module 294.

Control processor 436 controls the magnitude of the current sourced andsunk by sources 430 and sinks 432 and to which the electrodes thesources and sinks are attached based in part, on the instructions loadedin the processor. While not shown as a separate component, internal tothe processor 436 is a memory. This memory stores the operatinginstructions for the processor 436. This memory also stores theinstructions that are executed by the processor 436 regarding whichelectrodes 292 are to be tied to a current source 430 or sink 432 andthe magnitude of the current that is to be sourced or sunk by thecurrent source 430 or sink 432. These instructions may be received fromthe modulator/demodulator circuit or from the conductors that connectassembly 290 to the IDC 428.

The control processor 436 also asserts output signals based on themeasured voltages across the electrodes 292. To facilitate this feedbackcontrol, module 294 also includes one or more analog to digital voltageconverters (ADCs) 438, (two shown). The ADCs 438 are connected to theelectrodes 292 over a feedback matrix 162, also part of drive module294. In the illustrated version of the invention, feedback multiplexer440 is able to simultaneously connect any of the two electrodes 292 to aseparate one of the two ADCs 438.

Once electrode array assembly 290 is assembled, the assembly 290 isfolded. Specifically, the assembly 290 is bent around two fold lines 446and 448 that are parallel to and laterally spaced from the longitudinalaxis of the assembly. In FIG. 35, the distal ends of fold lines 446 and448 are depicted as dashed lines. One of the fold lines, line 446, isthe line along which the tabs 318 associated with bridge 302 are spacedfrom the adjacent tabs 318 associated with bridge 304. Fold line 448 isthe line along which the tabs 318 that extend from bridge 304 areseparated from the adjacent tabs 318 that extend from bridge 306. Eachfold line 446 and 448 is likewise the line along which one of side edgesof assembly head 326 is separated from the adjacent shoulder 340.

As a consequence of the folding process, bridges 302 and 306 are eachfolded inwardly towards center located bridge 304 as seen in FIGS. 43and 44. During the folding process, the legs 308 and 310 and the beams320 that extend between the bridges 302, 304 and 306 are subjected tofolding. Also folded are the membranes 322 between the bridges 302, 304and 306. The bridges 302, 304, 306, the tabs 318 and the electrodes 292carried on the tabs are not subjected to folding.

Typically one bridge, in FIG. 44 arbitrarily bridge 302, is initiallyfolded inwardly towards bridge 304 304. More particularly, bridge 302 isfolded inwardly to rest on the surface of drive module 294 spaced frombridge 304. The second outlying bridge, bridge 306, is then foldedinwardly so that bridge 306 overlies first bridge 304, the drive module294 and then bridge 302.

Once the assembly 290 is so folded, the tabs 318 that extend frombridges 302 and 306 are both substantially in registration with the tabs318 that extend from bridge 304. This is seen best in FIG. 45 whereinonly the top most of the bridges, bridge 304 is seen when assembly 290is in the folded state.

It should be appreciated that, during this folding process, neitherdrive module 294 nor terminal pad 296 is subjected to appreciablefolding.

Once electrode array assembly 290 is so folded, the assembly 290 isfitted in the lumen of the deployment (inner) cannula 240 (FIG. 26).Since the assembly is folded, the assembly can be fitted into adeployment cannula with a lumen 242 that has a major diameter less thanthe unfolded width of the assembly. The deployment cannula with thefolded assembly 290 therein, is fitted in an access cannula 250 (FIG.27). Access cannula 250, like deployment cannula 240, has a lumen 251with a major axis that is smaller in width than the unfolded width ofelectrode array assembly 290. In versions of the invention that includea cable 427 attached to module 294, the cable 427 is understood toextend through cannulae 240 and 250 through delivery cannula.

Then, using the above described deployment method, the electrode arrayassembly and deployment cannula 240 are positioned over the targettissue. The deployment cannula 240 and folded electrode array assemblyare advanced from the insertion cannula over the tissue. Once the foldedassembly 290 is properly positioned, the deployment cannula is thenretracted back into the access cannula while the electrode arrayassembly is blocked from similar movement. As the deployment cannularetracts away from the electrode array assembly, the super elasticproperties of the assembly frame 350 cause the assembly to unfold. Moreparticularly, the potential energy stored in the folded over beams 320is released. The release of this energy is what unfolds bridges 302 and306 away from bridge 304. Electrode array assembly 290 thus unfoldstowards the tissue through which the therapeutic current is to beapplied. Owing to frame 350 having a curvature that at least generallycorresponds to the curvature of the underlying tissue, when the assemblyis unfolded, the individual electrodes 292 are in close proximity to theunderlying tissue. FIG. 46 illustrates electrode array assembly 290would appear when deployed over a portion of the dura 31 of a spinalcord 30.

As long as bridges 302, 304, and 306 of array 290 are in registrationwith each other, the array is flexible, it can bend along itslongitudinal axis. In other words, assembly 290 when in the folded stateof FIG. 45, can bend both in and out of the page or to the right orleft. The deployment assembly used to position array 290 may include asteering assembly, not illustrated and not part of the presentinvention, to direct the folded array 290 in position over the targettissue. This particular deployment assembly may include features otherthan the disclosed access and deployment cannulae 240 and 250,respectively.

During this deployment process, the rounded shape of the distal ends ofshoulders 344 minimizes the likelihood that these shoulders will catchon the adjacent tissue. Also during the deployment process, membranes324 reduce the incidence of tissue adjacent the sides of the assemblyfrom being scraped by the outermost tabs 318 of the assembly 290.

This reduces the potential of the tissue becoming damaged by these tabs318.

After the electrode array assembly 290 is deployed, unfolded, thepractitioner can make minor adjustments to the position of the array.The rounded shape of the array head 326 reduces the resistance of thearray 290 to this movement.

In some versions of the invention, at the time electrode array assembly290 is implanted into the patient, the IDC 428 is also implanted in thepatient. Typically the IDC 428 occupies a space of 20 cc and is locatedbelow the skin in a pocket of subcutaneous fat. The IDC 428 includes anantenna for receiving power and instructions from a programmer externalto the patient. Cable 427 over which signals are exchanged with theelectrode array assembly 290 is connected to the implantable devicecontroller 428.

Once the assembly 290 is in the unfolded, deployed, state, membranes 322minimize the likelihood of tissue growth between beams 320. Membranes324 similarly minimize tissue growth between the outermost tabs 318.Similarly, the insulating material extends outwardly from the innertabs, the tabs integral with bridge 304 and the inner directed tabsassociated with bridges 302 and 306. This insulating material reducesthe open space around these tabs so as to likewise reduce the tissuegrowth adjacent these tabs. The minimization of the tissue growthbetween these features of the assembly 290 when deployed reduces theextent to which the tissue growth could inhibit the removal of theassembly 290 if such action is needed.

V. Activation of Assembly

Electrode array assemblies 28 and 290 of this invention are designed sothat current can be simultaneously sourced and sunk between differentcombinations of electrodes 36 and 292, respectively. FIGS. 47B through47E are representations of the magnitudes of the currents sourced andsunk through a set of eight linearly spaced apart electrodes 292 athrough 292 h in different modes of operating the array 290. FIG. 47A isa reference key for FIGS. 47B through 47E that indicates which one ofthe electrodes 292 a through 292 h is, at a given instant serving as acurrent source or a current sink. In FIGS. 47B through 47E, the “+”symbol indicates the electrode is serving as a current source; the “−”symbol indicates the electrode is serving as a current sink. The scalarnumbers in FIGS. 47B through 47E indicate the relative magnitude of thesourced or sunk current.

In practice, each active electrode will instantaneously source or sinkcurrent in the range of 0.1 to 20 mA in magnitude. Since electrodes 36and 292 of this invention have base pads with a surface area of at least1 mm², the current density at the surface electrodes is below the levelsat which the materials forming the electrodes could breakdown. Providedproper pulse-by-pulse charge balancing, the current density is alsobelow the levels at which the current flow through the tissue couldcause damage to the tissue.

FIGS. 48 through 51 represent the density of the currents through thetissue of the spinal cord 30 as a consequence of the current beingsourced and sunk according to the patterns of FIGS. 47B through 47E,respectively. More particularly, these Figures represent the currentdensity during the leading phase of a current pulse. In FIGS. 48 through51, the spinal cord is depicted as number of planar layers. In FIGS.48-51, electrodes 292 a through 292 h are drawn as if they represent asingle column consisting of electrodes 292. This is one electrode lessthan the number of electrodes in each column of electrodes that extendfrom bridges 302,304 and 306.

In FIGS. 48-51, eight electrodes 292 in a column of electrode arrayassembly 290 are shown on the spinal cord dura 31. Dura 31 isrepresented as a solid thick line. Below the dura the spinal cord 30 hasa region 492. Region 492 contains the cerebral spinal fluid (CSF). Belowthe CSF, the spinal cord 30 includes a region 494 with white matter.Below the white matter 494, in the center of the spinal cord, is thegray matter 496. Both the white matter 494 and gray matter 496 areformed from the actual nerves of the spinal cord. In one application ofassembly 290, the current is flowed through specific regions of whitematter 494. The current is flowed through the white matter to cause thenerves forming the white matter to react in such a way that the whitematter nerves affect the function of the nerves forming the gray matter496.

In FIGS. 48-51, the lines represent lines of constant current density(Amps/meter²). For ease of illustration, in each of FIGS. 48-51 thecurrent density of only a few of these lines is identified withspecificity. The actual direction of current flow is independent of thethese lines of constant current density. The CSF in region 492 has arelatively low impedance. In comparison, the white matter of region 494is of relatively high impedance. Accordingly, given both the proximityof the CSF region 492 to the electrodes 292 and the relatively lowimpedance of the CSF, current density is greatest in this region. Thismeans that the majority of the current flow is through CSF region 492.Nevertheless, if the current flow is of sufficient amplitude, there willbe sufficient current flow through the white matter region 494 to causethe desired reaction of the nerves forming this region.

The lines of constant current density of FIGS. 48-51 indicate that thereis some current flow above the electrode array assembly 290 and byextension above the spinal cord 30. This section of the body typicallycontains fat, a high impedance tissue. Accordingly, while there is somecurrent flow through this tissue, the amount of this current isrelatively small. Given both the magnitude of this current and thenature of the tissue, this current flow through the tissue typically hasno appreciable effect on the patient.

FIG. 48 illustrates the intensity of the current that develops as aconsequence of current being flowed between only a single pair ofelectrodes, the arrangement of FIG. 47B. Thus, it should be appreciatedthat to achieve the below-described current flow patterns, electrode 292b is tied to one of the current sources 430, electrode 292 g is tied toone of the current sinks 432. Control processor 436 sets the activesource 430 and active sink 432 to, respectively, source and sink thesame magnitude of current. The current therefore flows from belowelectrode 292 b to below electrode 292 g.

Consequently, as a result of this current source/sink arrangement, itcan be seen that along a line of constant depth from the surface of thewhite matter region, between the driven electrodes, the current flow isof a constant density. Specifically the current flows through point 502,below electrode 292 b, point 504, between electrodes 292 b and 292 g andbelow point 506, are relatively close to each other. Points 502, 504 and506 are understood to be a constant depth below the outer surface of thewhite matter region. Such a current flow may be desirable if the therapyrequires that the current influence a relative large section of tissue.

FIG. 47C represents how current is sourced and sunk between theelectrodes to minimize current flow in the white matter region betweenthe electrodes. Here, one electrode, electrode 292 b serves as theprimary source electrode and electrode 292 g functions as the primarysink electrode. Thus, electrode 292 b is, through multiplexer 434,connected to one of the current sources 430. Electrode 292 g, alsothrough multiplexer 434, is connected to one of the current sinks 432.The electrodes on either side of primary source electrode 292 b,electrodes 292 a and 292 c, are, through the multiplexer 434, tied to asecond one of the current sinks 432. These electrodes function assecondary current sinks. Simultaneously, the electrodes on the opposedsides of electrode 292 g, electrodes 292 f and 292 h, are tied to asecond one of the current sources 430.

When current is driven during this process, control processor 436 setsthe current source to which electrodes 292 h and 292 g are connected tosource one-half the current that is supplied by the source 430 to whichelectrode 292 b is connected. The current sink 432 to which electrodes292 a and 292 c are connected are set to sink one-half the current thatis to be sunk by the sink 432 to which electrode 292 g is connected.When current is sourced/sunk in this mode of operation a first primarycurrent flow path is from electrode 292 b to electrodes 292 a and 292 c.A second primary current flow path is from electrodes 292 f and 292 h toelectrode 292 g. There is also some current flow at least in the CSFregion in the space below and between electrode 292 c and electrode 292f.

FIG. 49 illustrates the densities of the current flows that develop as aconsequence of the current being sourced and sunk according to thepattern of FIG. 47C. Here it can be seen that within the white matter,the regions below electrodes 292 b and 292 g, points 512 and 516,identical to points 502 and 506, respectively, there is less currentflow than when the current flow was in the above-described patternassociated with FIG. 47B. Further, at point 514, identical in locationto point 504 representative of the region between the electrodes, thereis essentially no current flow.

FIG. 47D represents the source/sink configuration of the electrodes ofassembly 290 of this invention when it is desirable to flow currentthrough two regions of the spinal white matter while minimizing currentflow between these regions. Here electrodes 292 c and 292 e are set toserve as, respectively, the primary source and sink electrodes. Thus,electrode 292 c is connected to the current source 430 set to source thelargest magnitude current while, simultaneously electrode 292 e isconnected to the current sink 432 set to sink the largest magnitudecurrent. Electrodes 292 a and 292 b, the electrodes located to one sideof electrode 292 c, are connected to a second current source 430. Thissecond current source is set so electrodes 292 a and 292 b each sourceone-half the current sourced by electrode 292 c. Thus, in thisconfiguration of array 290, electrodes 292 a and 292 b function as thesecondary current source electrodes. In this configuration of theinvention, electrodes 292 g and 292 h are simultaneously tied to asecond current sink 432. Processor 436 sets this second sink 432 so itsinks from each electrode 292 g and 292 h one-half the current that issunk by electrode 292 f.

When assembly 290 is operated in this configuration current flow throughthe white matter region is primarily through two spaced apart regions.One region is the region through which the current flows from electrodes292 c to electrodes 292 a and 292 b. The second region is the regionthrough which the current flows from electrodes 292 g and 292 h toelectrode 292 f. Points 522 and 526 on FIG. 50 are, respectively,representative of these first and second regions. Points 522 and 526 areanalogues to points 512 and 516 of FIG. 49. It can be seen that thecurrent flow through points 522 and 526 is greater than the current flowthrough points 512 and 516.

The above discussed current through the primary regions of the whitematter causes small currents to flow in opposed directions on opposedsides of point 522. Given the magnitude and opposed directions of thiscurrent flow, the current flow through point 524, analogues to point514, can be considered zero.

FIG. 47E represents how the electrode array assembly 290 of thisinvention can be configured to induce appreciable current flow in thetissue adjacent one of the primary source sink electrodes but not thecomplementary electrodes. In this configuration of the invention, twoelectrodes, electrodes 292 a and 292 b simultaneously function as theprimary source electrodes. Thus both electrodes are connected to acurrent source that sources the same current to both electrodes. Also inthis configuration of the invention, a single electrode, electrode 292g, functions as the sink electrode. Processor 436 therefore connectsthis electrode to one of the current sinks 432. Further in thisconfiguration of the invention the electrodes on either side ofelectrode 292 g, electrodes 292 f and 292 h, function as secondarysource electrodes. Electrodes 292 f and 292 h are thus connected to adifferent source than the source to which electrodes 292 a and 292 b areconnected. The source to which electrodes 292 f and 292 h are connectedcauses each of these electrodes to source one-half of current sourced byeach of electrodes 292 a and 292 b.

From each of FIGS. 47B through 47E it should further be understood thatin preferred versions of this invention the current sunk by the sinkelectrodes is identical to the current sourced by the source electrodes.This is to prevent stray currents flowing through tissue.

As a consequence of the assembly 290 being operated as described in FIG.47E, the current can be considered sourced over a wide area but sunkover a narrower area. FIG. 51 depicts the density of these currentflows. Here it can be seen that in the white matter region in thevicinity below electrode and 292 b, around point 532 analogous to point502, there is some current flow. There is also current flow in the whitematter region between electrodes 292 d and 292 e, point 534, analoguesto point 502. The current flow through points 532 and 534 issubstantially equal. From FIG. 17 it can further be seen that whencurrent is flowed in accordance with the pattern of FIG. 47E, thecurrent density is more intense in the white matter region betweenelectrodes 292 f and 292 g, point 536, analogous to point 506. In otherwords, the current flow through point 536 is appreciably greater thanthe current flow through both points 532 and 534.

Thus, it should be appreciated that a feature of this invention is thatby selectively causing more than three of the electrodes of assembly 290to simultaneously serve as source or sink electrodes that both theregion in the spinal through which the current flowed can be targetedand the intensity of the current flow adjusted. This targeting andfocusing/diffusing of current flow is what results in the currentcausing the desired reaction of the nerves forming the white matterregion 494. Still another feature of this invention is that byestablishing the source and sink electrodes so that they are asymmetricrelative to each other, assembly of this invention can be configured sothat only a specific defined region of spinal white matter is subjectedto appreciable current flow. It should likewise be appreciated that theoperating mode of the which electrodes function as sources or sinks canbe reset throughout the time the array is disposed against the tissue.This allows the location of where in the tissue the current is flowingand the focusing/diffusing of the current to be changed if modificationis warranted by changes in the condition of the patient.

Further, upon initial implanting of the assembly 290 one can drivecurrent flow through a number of different regions of the underlyingtissue. This allows the practitioner and patient to determine throughwhich region of tissue the current flow offers the most satisfactorybenefits and/or tolerable side effects. Once this region is identified,the IDC 428 can be set to cause the array to drive current through thisregion of tissue.

Given that the electrode array assembly of this invention includes bothplural rows and plural columns of electrodes 292, it should beunderstood that this invention also makes it possible to focus thecurrent flow through tissue that is laterally offset from the source orsinking electrodes. This is illustrated in FIG. 52. Here, the individualelectrodes 292 of the assembly of FIG. 35 are illustrated as rectangularblocks. To be consistent with FIG. 1, each horizontal line of electrodes292 is analogous with one of the columns of electrodes of FIG. 35. Eachdiagonal line of electrodes 292, extending from the top most column isanalogous to a row of electrodes of FIG. 1. Current is sourced from orsunk to the blocks (electrodes) containing whole numbers. The numbersindicate the relative strengths of the currents sourced or sunk be eachelectrode 292. As before the “+” symbol indicates that the electrodefunctions as a source; the “−” symbol indicates that the electrodefunctions as a sink.

In the illustrated configuration, two electrodes 292 in the third rowfrom the left side of FIG. 52 function as the primary source electrodes.The electrode in the fourth column, (from the top) and eighth row fromleft is the primary sink electrode. The electrode in the fourth column,ninth row is connected to a current sink to serve as a secondary sinkelectrode. The seventh through ninth electrodes in the third and fifthcolumns serve as field focusing electrodes. These electrodes are locatedon the opposed sides of the primary and secondary sink electrodes. Thesefield focusing electrodes source current, as opposed to the sinking ofcurrent by the primary and secondary sink electrodes.

As a consequence of current being sourced and sunk through the arrayaccording to the pattern of FIG. 52, the region in spinal cord whitematter section in which current flow is focused is located slightlybelow and slightly to the right of the electrode that functions as theprimary sink electrode, represented by point 544 in FIG. 52. In thisFigure, the dashed lines represent current flow through the tissue. Thisillustrates that since the current can be sourced or sunk over a twodimensional area (discounting for curvature of the tissue) the regionsin which the current flow is focused can be both longitudinally andlaterally offset relative a given primary source or sink electrode.

FIG. 53 illustrates another feature of assembly 28 or 290. Here, byselectively sourcing and sinking current through two sets of spacedapart electrodes, current flow is simultaneously focused through two ormore spaced apart regions of the tissue against which the assembly isapplied. In FIG. 53, the electrodes are arranged in the same pattern asthe electrodes of FIG. 52.

In FIG. 53, the second column down/second row from the left electrodefunctions as the primary sink electrode for a first current flow path.The fifth column down/second and third row from the left electrodesfunction as the complementary electrodes that source the current sunk tothis first primary sink electrode. Simultaneously with the current beingflowed between the electrodes of this first set of electrodes, currentis flowed through a second set of electrodes. This second set ofelectrodes includes the second column down/sixth row from the leftelectrode functioning as the primary source electrode. Also part of thissecond set of electrodes is the third column down/eighth row from theright electrode functioning as the primary sink electrode.

Spaced apart bridges 302,304 and 306 of assembly 290 of this inventionallow the assembly to be fitted in a deployment cannula that is smallerin width the width of the unfolded assembly. This allows the assembly tobe positioned percutaneously, through the access cannula, or using otherminimally invasive surgical techniques. In addition to allowing theassembly to be folded, bridge 302, 304 and 306, when in registrationwith each other, allow the assembly to flex. This facilitates theprecise positioning of the assembly 290. Moreover, in versions of theinvention constructed as described above, the bridges are narroweradjacent the distal end of the assembly in comparison to the proximalend. This feature of the invention provides the assembly with moreflexibility adjacent the distal end, this being the end of the assemblyin which flexibility is most useful for positioning the assembly.

While bridges 302,304 and 306 are, as a result of the folding process,positioned to overlap, neither the bridges themselves nor the associatedelectrode carrying tabs 318 are subjected to folding. This makes itpossible to construct the electrodes 292 of the assembly 290 of thisinvention with the iridium layers having the necessary thickness withoutrunning the risk that appreciable folding of these layers will result intheir breakage.

Further, the sealing of drive module die 408 in the outer capsulemechanically isolates both the die and the underlying terminal pad 296.This prevents the rivets that connect the die to the terminal pad frombeing exposed to excessive strain during the array folding/bendingprocess. This strain, if excessive, could fracture the rivets.Post-deployment, the outer capsule prevents biological or chemicalattack on components fabricated on the die 408.

Membranes 322 and 324 prevent damage to the tissue surrounding theassembly 290. The membranes 322 and 324 also inhibit tissue growthadjacent and between the features of the assembly. The minimization ofthis tissue growth reduces the likelihood such tissue growth couldinterfere with the removal of the assembly 290.

VI. Alternative Embodiments

It should be understood that the foregoing is directed to specificversions of this invention. Other versions of this invention may havefeatures different from what has been described. For examples, thefeatures of electrode array assemblies 28 and 290 can be selectivelycombined with each other.

It should be appreciated that assemblies 28 and 290 of this inventionmay have applications other than for implantation adjacent the spinalcord dura. For example, the assemblies can be implanted against thebrain, the vagus nerve, sacral nerves, phrenic nerves, thalamus or othernervous system structure. In these versions of the invention, thestructural features of the assembly may have dimensions different fromwhat has been described. For example, by increasing the width of beams320 one could increase the potential energy the beams release during theunfolding process. Providing an array assembly of this invention withwide-width beams may be useful if the assembly is intended fordeployment adjacent tissue that is highly resistant to such deployment.Likewise, the thicknesses of the components may be different from whathas been described.

In some versions of this invention, it may not be necessary to providethe frame. Similarly, there is no requirement that in versions of theinvention with a frame, that the whole of the frame be formed fromsuperelastic material. Thus, in some versions of the invention, it maybe necessary to only form the beams 320 of the superelastic material.

For example, there is no requirement that all versions of electrodearray assembly have three bridges. Some versions of the invention mayjust have one or two bridges while other versions of the invention havefour or more bridges. In versions of the invention with two bridges, onebridge may extend from the assembly terminal pad. The second bridge isparallel to and laterally spaced from the first bridge. To fit theassembly 290 in the deployment cannula, the second bridge is folded overthe first bridge.

In versions of the invention with an even number of four or morebridges, equal number of bridges may extend from opposed sides of theterminal pad. The bridges are folded together so that one face of onebridge is folded toward the adjacent face of the opposed bridge. Thus, afold of four bridges may appear as the letter “W”. In versions of theinvention with an odd number of five or more bridges, there may be acenter bridge that extends from the terminal pad. The other bridges arefolded over or under the center bridge. Alternatively, the bridges onone side of the center bridge are folded first so as to be close to thecenter bridge. The bridges on the other side of the center bridge arethen folded so as to be disposed over the bridges closer to the centerbridge.

Likewise, there is no requirement that in all versions of the invention,the widths of the bridges decrease from the location at which the drivemodule is attached to the assembly. Furthermore, in some versions of theinvention, the widths of the bridges may change along their lengths forreasons other than the number of conductors present on the bridges.Thus, some bridges may have relatively wide sections where it is desiredto minimize array flexibility and other sections of narrow width whereit is desirable to increase flexibility. In some embodiments of theinvention bridge sections of narrow width and wide width may beinterleaved with each other.

Regardless of the number of bridges or rows and columns of electrodes,it should be understood that in most versions of the invention, theassembly includes at least 16 individual electrodes and in many versionsof the invention at least 30 electrodes.

Other variations in the geometry of the array are also possible. Forexample, in the described version of the invention there are two beams320 between each row of electrodes 292 (tabs 318). In some versions ofthe invention there may only be a single inter-bridge beam or three ormore inter-bridge beams between each row of electrode-carrying tabs 318.Similarly, there is no requirement that the number of inter-bridge beamsbe symmetrical across the array or identical along the length of thearray. In some versions of the invention, these beams may even extenddiagonally between the bridges. Such variations may be desirable toregulate the extent the potential energy released by the beams forcesthe unfolding of the array.

Similarly, there is no requirement that in all versions of theinvention, the electrodes only be located on one side of the assembly.In some versions of the invention, electrodes may be disposed on allsides of the substrate. In these versions of the invention, the assemblycould be considered to have opposed active sides.

The materials from which assemblies 28 and 290 is formed may also varyfrom what has been described. For example, the actual material of eachelectrode that serves as the interface material with the adjacentmaterial may not always be iridium. In some versions of the invention,this material may be iridium oxide, platinum, an alloy of platinum, aplatinum oxide or other conductive biocompatible material.

In some versions of the invention, adhesives as opposed to mechanicalconnecting members hold the die 408 to the terminal pad 296.

Similarly in some versions of the invention, manufacturing methods maydictate that the outer capsule shell 418 and cap 420 be formed as asingle molded-in-place structure.

Further, there is no requirement that in all versions of the inventionelectrode-carrying tabs extend from both sides of each bridge. In someversions of the invention, it may be desirable that only a single columnof electrode-carrying tabs extend from a single side of one or morebridges. Typically, it is one or both of the outermost bridges, thebridges located furthest from the terminal pad, that include only asingle column of electrode-carrying tabs.

Similarly, alternative methods may be used to manufacture the assemblies28 and 290 of this invention may be different from what has beendescribed. For example, the method of manufacture disclosed in U.S. Pat.App. No. 61/057,684, filed 30 May 2008, published as U.S. Pat. Pub. No.US 2009/0293270 A1 and PCT Pub. No. WO 2009/155084 A1, the contents ofwhich are incorporated herein by reference discloses one suchalternative method of manufacture.

Likewise, the number of rows of electrodes 292 may be fewer or greaterthan in the described embodiment.

Likewise, the structure of the drive module is understood to beexemplary, not limiting. In alternative versions of the invention, thedrive module may include less or more sub-circuits than in the describedversion of the invention. For example, in some versions of theinvention, the only circuit in the drive module 294 may be amultiplexer. In these versions of the invention, the implantable devicecontroller 428 does more than supply the current that is flowed throughthe tissue. The IDC 428 also outputs the instructions that cause themultiplexer to route the current to/from the specific electrodes 292.

In some methods of implanting the electrode array assembly of thisinvention, a stylet may be used to puncture the skin so as to form aportal into which the access cannula is inserted.

It should likewise be understood that there is no requirement that theelectrode array assembly of this invention always be implantedpercutaneously. Other procedures may be used to deploy this electrodearray assembly near the target tissue through which the current is beflowed. Thus, in some cardiac application where the array is appliednear the heart or neurological applications where the array is to beapplied adjacent the brain other procedures such as minimally invasivesurgical procedures may be used to implant the array.

Similarly, in some versions of the invention the IDC 428 may include abattery that is not rechargeable. In these versions of the invention aminor surgical procedure may be needed to replace the battery if not thewhole of the IDC.

Also, while in the disclosed version of the invention, the electrodearray assembly is bent, folded to be fitted in the insertion tool, thatmay not always be the case. In some constructions of the invention, toreduce the width of the electrode assembly, the assembly may be rolled.Thus the folding or bending of the electrode array should be interpretedinclude the rolling of the array, Again, the electrodes may be locatedon tabs that are not subjected to the degree of rolling to which thesurrounding portions of the assembly are rolled.

It should similarly be appreciated that deployment assembly of thisinvention including deployment cannula 240 within access cannula 250 maybe used to deploy electrode arrays other than the disclosed electrodearrays. Thus, cannulae 240 and 250 may be used to deploy and properlyorient an electrode array that, upon retraction of the deploymentcannula 240 does not unfold or unbend.

Likewise the deployment cannulae 240 and 250 may have different shapesthan what has been described. These cannulae can have lumens that havecircular or non-circular cross sections that may or may not vary inprofile along the length of the cannulae. Further, there is norequirement that the inner and outer delivery cannulae of this inventionall be constructed so that the cannulae lumens in which the electrodeassemblies are seated are open at both ends.

Accordingly, it is the object of the appended claims to cover all suchvariations and modifications that come with the scope of the belowclaims.

1-22. (canceled)
 23. An electrode array and deployment assembly, saidassembly including: a cannula consisting of a structural member formedfrom material that is insertable in living tissue, said structuralmember shaped to define a lumen having a major diameter and a distalend, the lumen open at the distal end of said structural member; and anelectrode array disposed in the lumen of said access cannula, saidelectrode array including: a frame, said frame having at least twolongitudinally extending bridges that are located side by side, at leastone beam that extends laterally between adjacent first and second saidbridges, at least one first tab that extends laterally from the firstsaid bridge towards the second said bridge, said tab being separate fromsaid at least one beam and the second said bridge, wherein said frame,when in an unfolded state, has a width greater than the major diameterof the lumen of said access cannula; and an electrode disposed on saidat least one first tab wherein, said electrode array is folded so as tobe disposed in the access cannula lumen, and when said electrode arrayis folded, said frame is folded around said at least one beam and saidat least one first tab is less folded than said folded beam.
 24. Theelectrode array of claim 23, wherein: said frame is further includes atleast one second tab that extends from the second said bridge, said atleast one second beam being separate from said at least one first taband said at least one beam, said at least one first tab and said atleast one second tab being positioned on said bridges so that, when saidelectrode array is folded, said at least one first tab and said at leastone second tab at least partially overlap; and an electrode disposed onsaid at least one second tab.
 25. The electrode array and deploymentassembly of claim 23, wherein said access cannula is further formed sothat said lumen has a minor diameter, the minor diameter being less thanthe major diameter.
 26. The electrode array and deployment assembly ofclaim 23, wherein at least said at least one beam is formed fromsuperelastic material.
 27. The electrode array and deployment assemblyof claim 23, wherein said frame is formed from superelastic material.28. The electrode array and deployment assembly of claim 23, wherein:said frame is formed to have two said beams that extend between thefirst one of said bridges to the second one said bridges, said beamsbeing longitudinally spaced from each other along said bridges; and saidat least one tab extends from said bridge at a location between saidbeams.
 29. The electrode array and deployment assembly of claim 23,wherein: said frame is formed to have at least one second tab thatextends laterally away from the first said bridge and away from thesecond said bridge; and a said electrode is disposed on the at least onesecond tab; and an electrode is disposed on said at least one secondtab.
 30. The electrode array and deployment assembly of claim 23,wherein: a plurality of said first tabs extend laterally away from thefirst said bridge to the second said bridge; sat at least one beam thatextends between the first and second said bridges is located between twoadjacent said first tabs and is separate from tabs; and separateelectrodes are disposed on said first tabs.
 31. The electrode array anddeployment assembly of claim 23, wherein said electrode is disposed overa surface of said frame that forms said tab.
 32. An electrode array anddeployment assembly, said assembly including: a cannula consisting of astructural member formed from material that is insertable in livingtissue, said structural member shaped to define a lumen having a majordiameter and a distal end, the lumen open at the distal end of saidstructural member; and an electrode array disposed in the lumen of saidaccess cannula, said electrode array including: a frame, said framehaving: at least three longitudinally extending bridges that are locatedside by side and spaced apart from each other; at least one first beamthat extends laterally between adjacent first and second said bridges;at least one second beam that extends laterally between adjacent secondand third said bridges; at least one first tab that extends laterallyfrom the first said bridge towards the second said bridge, said at leastone first tab being separate from said at least one first beam and thesecond said bridge; and at least one second tab that extends laterallyfrom the second said bridge towards the third said bridge, said at leastone second tab being separate from said at least second beam and thethird said bridge, wherein said frame, when in an unfolded state, has awidth greater than the major diameter of the lumen of said accesscannula; an electrode located on said at least one first tab and anelectrode located on said at least one second tab; wherein, saidelectrode array is folded so as to be disposed in the access cannulalumen, and when said electrode array is folded, said frame is foldedaround said at least one first beam and said at least one second beamand when said electrode array is folded, said at least one first tab isless folded than said at least one first beam and said at least onesecond tab is less folded than said at least one second beam.
 33. Theelectrode array and deployment assembly of claim 32, further including:at least one third tab, said at least one third tab being integral withthe second said bridge and extending towards the first said bridge, saidat least one third tab being separate from the first said bridge, saidat least one first tab and said at least one first beam, wherein whensaid at least one first beam is folded, said at least one third tab isfolded less than said at least one first beam; and an electrode disposedon said at least one third tab.
 34. The electrode array and deploymentassembly of claim 32, wherein said at least one first tab extends fromthe first said bridge and said at least one third tab extends from thesecond said bridge at locations so that, when said at least one firstbeam is folded, said at least first tab and said at least one third tabat least partially overlap.
 35. The electrode array and deploymentassembly of claim 32, wherein: a plurality of said first beams extendfrom the first said bridge to the second said bridge, said first beamsbeing spaced apart from each other along said bridges; and said at leastone first tab extends from the first said bridge at a location betweensaid first beams and is spaced apart from said first beams so that, whensaid first beams are folded, said at least one first tab is folded lessthan said first beams.
 36. The electrode array and deployment assemblyof claim 32, wherein at least said at least one beam is formed fromsuperelastic material.
 37. The electrode array and deployment assemblyof claim 32, wherein said frame is formed from superelastic material.38. The electrode array and deployment assembly of claim 32, whereinsaid electrodes are disposed over surface of said frame that forms saidtabs over which said electrodes are located.
 39. The electrode array anddeployment assembly of claim 32, wherein: said cannula with said foldedelectrode array disposed therein is able to flex, so that said cannulafunctions as a deployment cannula; and said deployment cannula, withsaid folded electrode array disposed therein, is disposed in an accesscannula that is separate from said deployment cannula.
 40. The electrodearray and deployment assembly of claim 32, wherein: said cannula withsaid folded electrode array disposed therein is able to flex, so thatsaid cannula functions as a deployment cannula; and said deploymentcannula, with said folded electrode array disposed therein, is disposedin an access cannula that is separate from said deployment cannula. 41.An electrode array and deployment assembly, said assembly including: acannula consisting of a structural member formed from material that isinsertable in living tissue, said structural member shaped to define alumen having a major diameter and a distal end, the lumen open at thedistal end of said structural member; and an electrode array disposed inthe lumen of said access cannula, said electrode array including: aframe, said frame having: at least two longitudinally extending bridgesthat are located side by side, at least two beams that extends laterallybetween first and second adjacent said bridges, said beams being spacedapart from each other along said bridges; and at least one first tabthat extends laterally from the first said bridge towards the secondsaid bridge, said tab being located between two said spaced apart beamsand being separate from said beams and the second said bridge, whereinsaid frame, when in an unfolded state, has a width greater than themajor diameter of the lumen of said access cannula; and an electrodedisposed on said at least one first tab; wherein, said electrode arrayis folded so as to be disposed in the access cannula lumen, and whensaid electrode array is folded, said frame is folded around said beamsso that the first said bridge and the second said bridge at leastpartially over lap and, when beams are folded, said at least one firsttab is less folded than said beams.
 42. The electrode array anddeployment assembly of claim 41, wherein: at least one second tabextends laterally from the second said bridge; and an electrode isdisposed is disposed on said at least second tab.
 43. The electrodearray and deployment assembly of claim 41, wherein: at least one secondtab extends laterally from the second said bridge towards the first saidbridge, said at least one second tab being separate from said at leastone first said tab and said beams, so that, when said beams are foldedsaid at least one second tab is less folded than said beams; and anelectrode is disposed on said at least one second tab.
 44. The electrodearray and deployment assembly of claim 41, wherein: at least one secondtab extends laterally away from the first said bridge, said at least onesecond tab extending from the first said bridge in a direction oppositethe direction in which said at least one first tab extends; and anelectrode is disposed on said at least one second tab.
 45. The electrodearray and deployment assembly of claim 41, wherein: said frame includes:at least three said bridges; a first set of said beams that extends fromthe first said bridge the second said bridge; a second set of said beamsthat extends from the second said bridge to a third said bridge; and atleast one second tab that extends outwardly from the third said bridge,said at least one second tab being separate from the second set of saidbeams; and an electrode disposed on said at least one second tabwherein, said at least one first tab is located on the first said bridgeand said at least one second tab is located on the third said bridge sothat, when the first and second sets of said beams are folded so as tofold said electrode array, said at least one first tab and said at leastone second tab at least partially overlap.
 46. The electrode array anddeployment assembly of claim 41, wherein said electrode is disposed overa surface of said frame that forms said at least one tabs over whichsaid electrode is located.
 47. The electrode array and deploymentassembly of claim 40, wherein: said cannula with said folded electrodearray disposed therein is able to flex, so that said cannula functionsas a deployment cannula; and said deployment cannula, with said foldedelectrode array disposed therein, is disposed in an access cannula thatis separate from said deployment cannula.